I really like the high concept here. This seems like a good way to mitigate alpha strikes somewhat and to make the flow of the game more interesting. That said, I do have few questions and thoughts.
Units are added first to the Recon Element and then to the Battleline Element in stages. The player must add all units from their army that fit within a particular stage before adding any units from the next stage.
Is this section in reference to things like summoned units? And if so, how are such units treated in terms of being able to activate, etc.? So for instance, my recon element contains about 24% of my army and a character that can summon daemons. He summons some daemons on turn 1. Are those daemons treated as being part of the recon element? And if not, are they still allowed to charge, fight, etc on turn 1? If my battleline element is about 49% of my points and the summoned daemons are about 10% of my points, do the daemons get bumped back into the reinforcement element? Do they have to stand around doing nothing until turn 3? Do they get added into the later elements instead of actually being summoned onto the table? Just need some clarification.
Also, when do you break your list up into elements? I suggest making it happen during list creation to avoid lots of hemming and hawing and calculating points during deployment.
While your guidelines for what can fit into each stage are pretty flavorful, I feel like you're setting yourself up to miss some weird outliers. Would it be better to simply add a "recon" and "battleline" keyword to all appropriate units? You can use your current criteria as a guideline for what units should get those keywords and then edit the resulting list manually, but it's probably easier to explain to people in the long run. Easier to glance at a list of "recon" units than to go through the long list of criteria too.
-Any unit with the Fast Attack battlefield role
-Any unit with a rule allowing it to be deployed on the battlefield outside the normal deployment zone, or any unit with a rule allowing it to move prior to the beginning of the first turn
While fluffy, I feel like this could mess up how many such units are actually utilized. Drukhkari scourges, for instance, are a fast attack unit that generally wants to deepstrike in, sucker punch a unit, and then pray it somehow stays alive to shoot again (but usually doesn't.) If I'm forced to put scourges into my recon element (by virtue of not having enough other stuff that satisfies the stage 1 criteria), then I'm forced to put a fragile suicide unit on the table at the start of the game, possibly out of range with its weapons, and subject to the anti-shooting benefits of Night Fighting. Basically any "deepstrike and shoot" unit gets hurt by this.
Although I guess I could just put them in the reinforcements element? That would delay my delivery of them until turn 3, but maybe that's a feature rather than a bug.
-Optional: Any unit with a rule allowing it to be deployed off of the battlefield. Any such unit placed in the Recon Element loses that rule and must be deployed normally.
I don't think I like this. I might want to start my mandrakes or swooping hawks on the table, but I don't want to give up their ability to re-enter reserves to do so. And something like nurglings feel like they lose a lot of value if they have to jog all the way to an objective you'd normally start them on. Also, drop pods.
-Any unit with the Troops Battlefield Role
-Any unit with the HQ Battlefield Role
-Any unit with the Elites Battlefield Role
I can think of troops and elites that seem like they really ought to be in the recon element. (Eldar rangers, striking scorpions, kommandos, genestealers, arguably skitarii rangers, anything drukhari if it happens to be in a venom. Probably anything described as sneaking up behind the enemy and quietly stabbing them.)
Not allowing HQs
to deploy with your recon elements does feel weird in some cases. My fast attack swooping hawks can be in my recon detachment, but Baharroth, the first of his aspect, lord of the swooping hawks, has to wait his turn to arrive. A marine captain with a jump pack can't deploy next to your jump pack assault marines. Darkstrider(?) (the tau guy) can't deploy with his pathfinder buddies.
Night Fight: All units are treated as receiving the benefits of Light Cover and Heavy Cover during battle round 1
Feels like this should probably be the benefits of Dense cover (-1 to hit) instead of light/heavy cover. The darkness makes you harder to hit, but doesn't make your armor thicker.
The first player sets up their Battleline Element on their deployment zone battlefield edge,
I could be mistaken, but I don't think "deployment zone battlefield edge" is currently a defined thing in 40k
. And in some missions (Open War and Narrative mostly), one player might have a deployment zone that doesn't touch a table edge at all or one that covers two table edges enough to make which edge is "their" edge ambiguous.
Additionally, this does potentially short-ranged units quite a bit if your battlefield edge is one of the short edges (meaning you have to travel further across the table to get close to the enemy). I'm picturing something like thunderwolf cavalary. I get that the point is to slow down the pace of the game, but being forced to deploy so far back and thus giving your opponent more time to shoot at you is a bit of a double-whammy. Perhaps it would help if units could come in any where within their deployment zones so long as they're touching any board edge. So the T cav wouldn't be pulling off turn 1 charges, their opponent would need to be a little cautious to avoid turn 2 charges.
Battle Round 2:
-The first player takes their entire turn with their Recon Element
-The first player sets up their Battleline Element on their deployment zone battlefield edge, and then takes the Command, Movement and Psychic phases with their Battleline Element
-The second player takes their entire turn with their Recon Element
-The second player sets up their Battleline Element on their deployment zone battlefield edge, and then takes the Command, Movement, and Psychic phases with their Battleline Element
-The first player takes their Shooting, Charge and Fight phases with their Battleline Element
-The second player takes their Shooting, Charge and Fight phases with their Battleline Element
How does the Fight phase work with multiple elements? Say my recon unit is locked in combat with your recon unit at the start of the turn. Then my battleline unit charges your recon unit. Assuming your recon unit survives everything, that mean it gets to fight during my recon element's fight phase, your recon element's fight phase, and again during my battleline element's fight phase? (And potentially then again on your battlelien element's fight phase?) If so, that's a lot of potential bonus attacks for a given round of gameplay.
Alternatively, if your intention is that all units only fight during their own element's fight phase, does that mean that you're getting rid of all the fight first/last rules in the game? I'm probably overlooking something.
Also, I worry that this way of doing things is going to be really rough on short-ranged deepstrikers. Let's say I'm playing a scions army. I have a lot of short ranged special weapons that need to get within 12" of my target to be effective (things like plasma and melta guns). The way I see it going is...
1.) My opponent puts his battleline units somewhere.
2.) I put my scions within 12" of the thing they want to shoot at.
3.) My opponent shoots and/or charges my scions.
4.) My scions are dead having never fired their weapons.
Battle Round 4, 5, and 6
I haven't been able to play in a few months, so I'm genuinely a little fuzzy, but is turn 6 a thing this edition? Either way, just a nitpick. Your intention is clear.
Overall, I like the general idea, but I think some of the limitations and alternating activation rules might actually be creating more problems than they're solving. Some suggestions:
* Get rid of all or most of the criteria for which units can go into the recon and battleline elements. Maybe create a keyword for this purpose as suggested above. I like the general idea of representing "recon units" being recon-y, but your current criteria keeps me from putting some of my fast and sneaky units into recon and risks making some of my fast attack units less good at being fast/recon units.
* Probably ditch the alternating activations thing. It creates the confusion/problems I pointed out above. I'm sorry to advise against it; I'm sure you put a lot of thought into it.
* Instead, maybe just don't let units shoot or charge on the turn their element arrives? So player 1 puts down models basically announcing their intent to start wrecking stuff in one general area, and then their opponent places models with knowledge of the threat range of the element player 1 just deployed. So you end up with the staggered threat that seems to be the main point of this proposal, but you don't have to add a lot of complication (that might turn off potential users of these rules) via the alternating activations. You get to avoid answering some of my above questions too.
* As an exception to the above bullet point, DO
let units with a deepstrike type rule shoot/charge on the turn their element arrives. This is a decent way to distinguish them as being fast/sneaky/ambushy.
* I'm not sure how compatible these rules actually are with certain other missions (mostly thinking Open War and Narrative missions), so it may be worthwhile to create a list of actually compatible missions or even to (down the road) create some missions designed with this ruleset in mind.
And as a final thought, does this ruleset possibly make things like Mortarion too powerful? Big bully units that you really need your whole army's firepower to take out? If most of my big guns aren't shooting until turn 2 or 3, I feel like there's a good chance Mortarion or Magnus will still be around at the end of the game. Not sure if that necessarily means I lose though. Similarly, mission objectives that require you kill certain units might be harder to pull off if you're missing a lot of your guns until the second half of the game.
Overall, a great start and a cool idea.