The majority of the posts I've placed in Proposed Rules have focused on "general purpose" issues rather than on a codex-per-codex basis. I can drudge up several ideas, but notable ones included:
-"Big three": rules of
40k. ITC Invisibility (in my base rewrite i recommend making it work like Stealth in Warmachine), cannot "modify" an Invulnerable save to better than 3++ (stuff like Shadow Fields are fine, but Grimoire + Cursed Earth are not), and removing Deathblow from D in exchange for it being a -2 penalty to Invulnerable Saves.
-Reworking the 7e Psyker system: it's the same one lord_blackfang used for potica edition

Basically: More Warp Charge but no more pooling, and rather than minimum casting values, powers have degrees of success. Deny the witch, rather than being all-or-nothing, can subtract successes from a power, so even if you're hit by a nasty Witchfire that's
AP 3, getting only one denial could reduce it to
AP 4, etc.
-Reworking Overwatch to be based on 2nd ed declaration (forfeit shooting and further movement), with 3rd-7th targeting restrictions. The key difference is rather than triggering on enemy movement, it would trigger on enemy declaration of attempt to attack a unit near the Overwatching unit. This would also replace Interceptor incidentally.
-Removal of instant death and vehicle destroyed - Explodes with massive damage (instant loss of D3
HP/wounds). Replacing strength double toughness with strength >= toughness + 3, making the vehicle damage mods start at
AP 3, and adding extra wounds/
HP to compensate.
-Removal of Salvo weapons.
-Removal of Sweeping Advance with "Free Strikes"
-Reworking Stomp. Rather than D3 blasts with a
d6 chart, it's one blast at the user's strength and
AP 2. You can do up to 2 extra blasts, but each extra blast means they all have -2 Strength and -1
AP. Stomps must be a minimum of S6, all Stomps must be within 2" of the stomper, and you cannot place multiple stomp blasts on top of each other.
-Summoning in 7e requires forfeiting all movement.
-Allowing Death or Glory vs Vector Strikes. Smack that Heldrake or Flyrant out of the sky if it dares to hit your Assault Terminators!
-Terrain is just "half movement unless MTC".
-Shooting is based on comparing
BS vs target initiative. Cover increases initiative for defenders being charged.
One thing I'm currently brainstorming is how to rework Tank Shock. One thing that is irksome is that only Tanks can push models around and not all Monsters, and that a Land Raider can be stopped in its tracks by a flimsy Warbuggy but can shove a Trygon out of the way (or kill a Stormsurge). One thing I was thinking of doing was merging Tank Shock, Ram, and Hammer of Wrath into the same rule, and giving vehicles a strength value. Comparative strength determines whether you can displace models, or whether it's a simple collision: For example, a Rhino might be able to scatter Guardsmen around, but a wall of Bullgryns would be impassable. Inversely, said Bullgryns could push other infantry off objectives but would get shoved aside by a Carnifex.
Another thing I want to work on is "war rig" rules, making "Chariot" and open-topped mergable into each other.
As for army-building:
-I am torn on formations. On one hand you have the lazy spamming ones (Aspect Host, Riptide Wing, etc) and on the other hand you have the ones that are bland and useless (Khorne's Bloodstorm, etc) and the ones that are *fun* (Demolition Claw, Helforged Warpack, etc). If the *fun* bonuses could be extracted from those formations I would be cool with them going the way of the dodo, but I do like the idea of formations that give an appreciable bonus for an appreciable opportunity cost as while certain units may be usable in a vacuum, armies that are certain compositions may lag behind others. While Gladius may be the way to play Marines, let's not kid ourselves that stripping it wholesale would probably swing things back in favor of Gravbikes.
-The alternative idea was doing a "flexible FOC" similar to what 8th is doing. Take a
CAD, scale by points, and decide what you want in your army. Rather than Lord of War, I'm OK with certain "power units" counting as "Relics of war", meaning they take up an extra slot (or several) within their respective category. I've gone back and forth on the idea of splitting
HQs into Lords and Heroes ("the Marbo Slot"

.
-Characters are "built" similar to Forgeworld
HQs, having a base profile and a series of sub-specializations (think like Corsair Princes, Legion Centurions, etc). As an example, Fireblade would be the catchall for non-suit Tau leaders, but said character could be a Firesight Marksman, a Tank Veteran, or a Survivor (Darkstrider, Kais in Fire Warrior, etc).
-"Engineer HQs" can commandeer a vehicle of choice as their ride, allowing for fully armored lists.
-One thing I experimented with was "Special Mission Loadouts". Sometimes, there are certain equipment upgrades that are so specific that you never spend points on them, but they can be nice for niche situations. Grapple guns for scouts, fire barrels on Guard tanks, etc. The idea I had (and as an alternative to sideboards) was each unit has three choices of "free upgrades" to choose from, after army creation but before the game starts. Doing it for the entire army may be
OTT, but another alternative would be having a fixed number of units that can do it per point level (one per 500 pts) with certain
HQs (techmarines, etc) allowing more.
Tau:
-Aun'shi replaces Stubborn with Zealot, and replaces his stances with War Hymns (renamed "Prophet of the Greater Good"

. He's unique enough to serve as a "Battle Preacher" for the Tau while paying a premium for the privilege (and in an army not kitted for melee).
-Make the Riptide cheaper or tougher but cut its damage factor. Make it tanky rather than tanky and killy. The Heavy Burst Cannon simply becomes Heavy 6 (or 9 with overcharge), and the Ion Accelerator is just a regular Ion Cannon now, with Nova Charge making it
AP 2 (rather than it being
AP 2 on standard overcharge).
-The Nova Reactor is no longer "buff or wound." It's "buff this turn, cooldown and penalty next turn." So boosting your Invulnerable to 3++ on turn 1 means no Invulnerable on turn 2, etc.
-Smart Missile Systems only fire 2 shots but have Precision Shots and Rending (combine with Advanced Targeting if you wish). Hi-Yield Missile Pods are either Heavy 2 Blast.
-Tau Vehicles get "Drone racks" which are customizable replacements for "pair of gun drones".
-Still working on drone interchangability.
Eldar:
-Dire Avengers are 3+ armor, Jetbikes are 4+ armor. Scatter Lasers on Eldar Jetbikes are 18" (This combined with the revised overwatch rules and D could make them far more bearable).
-Guardians and Storm Guardians get a 1 pt reduction.
Chaos Space Marines:
-Chaos Plasma weapons have "Gets Really Hot." They fire an extra shot, but each roll of 1 counts as two rolls of 1 (meaning 2 saves, potentially 2
HP lost, etc)
-20 pts reductions for Daemonforged models. Baleflamer costs extra though. Daemonforged Models gain Power of the Machine Spirit (so Defilers aren't hosed). Defiler Battlecannon gains Indirect Fire.
-Redid the Thousand Sons, period.
Tyranids:
-The Maleceptor can "recycle" Warp Charge used by other non-Maleceptor Tyranid Psykers within Synapse Range. For each
WC roll a die, and on a 5+ it gains a token that it may use for that Psychic Phase alone. The Maleceptor knows every power in the Tyranid Discipline, and upon successfully manifesting a power may attempt to manifest it a second time; this does not count against the
-Venom Cannons are direct-fire again.
-Spore Mines have alternative payloads ala 4th. In addition to Biovores/Harpies/airdropping them, Lictors can also lay single Spore mines.
-I haven't thought as much about Nids
tbh.
Sisters:
-Fundamentally reworked them. I can expand on this.
Orks:
-Still in draft mode.
-Clan rules. Each Clan gives a minor bonus, and has recommended "clan gear" for modelling.
-Warboss grants Clan bonus to unit he leads, either doubling it if it's the same clan, or replacing the other bonus ("I say youz Goffs now so shut it!"

. A Waaagh is treated like the Warboss is in every unit, and they're all his clan.
-Big Meks can take a Speedsta.
-Plans for a "medium tank" between the Trukk and Battlewagon.
-Removing "Looted Wagons" and just providing a general list of "Looted Bitz" that can be used to kustomize vehicles as a whole.
-Removing Flash Gitz as a unit. Rather, any Nob can be a Flash Nob, allowing access to Kustom Jobs on any weapon they own.
Necrons:
-Early draft.
-Monoliths have a Power Matrix, which can be calibrated for different tasks. A Monolith may do one of three things at the start of its turn: Teleport a Necron unit ala 3rd ed (including out of
CC), fire the Particle Whip (now a primary weapon), or activate the Exile Gate (force an enemy unit to move 6" towards the gate, any model that makes it b2b takes a Destroyer Hit).
-Custom Cryptek Harbingers, and customizable C'tan.
-Necron
ICs cannot join units that do not start off with Reanimation Protocols (nerfing Orikan-stars)
Plenty more, feel free to ask about specifics.