I'm less familiar with orks than I am with most armies, but I do face them a fair bit. Some of these proposals seem reasonable, others seem fine but strange, and others seem like they're probably too good. I'll address the last two types. You can assume anything I don't mention seems either totally fine or even super-cool in my book.
Gretchin: Aren't gretchin with long-ranged guns just called "artillery"?
Burna Boyz: These guys already consistently do some pretty serious damage to me. Do they really need to be improved? They're generally taken in squads of 5 or 10, right? So you'd only be paying 20 points to make the squad capable of harming a wider variety of targets, wounding many
MCs on 5s, roasting everything T3 on a 2+, and ignoring the armor (and cover) of anything not in power armor or better. It's not that people couldn't deal with it (they already cripple most things they attack, and they'd still die just as easily as now), but is it really necessary for them? Maybe this would work better as part of a formation that has you take a trio of burna boyz or something so that if you jut really love flamers, you can be rewarded for your specialization. Or am I overestimating how effective they are currently?
Tank Bustas: This is another unit that seems like it's already pretty solid as it is. What's the issue you've encountered with them that makes you want to power them up? Also, wouldn't a range boost and a consistent number of shots equal to the average number of shots from a loota make them start to compete heavily with lootas? You'd give up a little bit of range and the possibility of more shots for a more consistent damage output, higher strength, and melee anti-tank.
Meganobz are only Toughness 4 by default, right? T5 seems like it's probably fine, though anything strength 3 is only going to be wounding you on 6s in challenges, which seems a bit much. T6 would probably be a bit overkill. Marines wounding on 5s seems reasonably fluffy, but 6s is rather nasty. Plus, you'll push people even further towards spamming lots of big guns rather than being willing to take you on in assault, which is probably less fun for everyone involved. Assuming they're T5 base with your changes, you'll potentially be able to make biker nobz tougher than Nurgle bikers with a pain boy. That's another thing that can be dealt with, but I'm not sure how much fun that is to face. Nurgle bikers are tough, but at least they aren't also packing a power claw each. It would put the unit in thunderwolf cavalry territory, only worse in some ways. You wouldn't have invul saves, but a jink save and
FNP is probably better in most situations anyway. I know *I* probably wouldn't enjoy playing against it much, but I'm not fond of death star units in general.
War bikers: "Skilled rider" seems a bit strange for an ork. "Skilled" isn't really the word that comes to mind when I think of an ork doing reckless stunts on a bike, but I guess you could probably fluff it as ork luck/craziness keeping him safe or something. Let me put this to you: if an ork on a bike has skilled rider, what bike unit in the game *shouldn't* have skilled rider? Maybe make skilled rider a bonus for a kult of speed formation/detachment? That way, if you're heavily-invested in bikers, you don't have to worry about losing a big chunk of guys to terrain accidents, and it helps to distinguish the "dedicated" bikers from average joe orks, thus preventing all orks on bikes from having a level of dexterity that even an eldar windrider can't match.
Charging after turbo-boosting is a cool idea, but it seems a bit much. You're basically saying bikers can assault anywhere on the table turn 2 provided they don't flub their charge roll. Maybe give them fleet on turns you waaagh so that their average charge is better instead? And again, this feels like something that would fit better as a deatchment/formation rule than as a default rule. Would nob bikers also get these benefits?
Also, why +1 strength? Is it to help with HoWrath attacks? If so, I'd probably rather just see them get a rule for +1 HoWrath attacks.
Koptas: See above about the bonus to strength. The extra rokkit shot thing would be fine if priced appropriately, but it shouldn't be a cheap thing. Koptas are already one of the better ork units out there (you can make an odd little death star with them), and their shooting is nothing to sneeze at. They're even one of the scarier units in the game to be overwatched by with their
TL high-quality guns. Do you really feel they need a boost? The ork players around here seem to run these guys with no complaints.
Lootas: No thanks. ^_^; I realize that lootaz are inconsistent, but they're pretty fantastic for their points. If you roll hot on your number of shots and/or to-hit rolls, you can put out a terrifying about of dakka. D3+3 shots would more than double their average damage output, and they're already making most things in the game sweat as is. +1 strength would be more reasonable, but then you might be stepping on tankbusta toes. I don't know. Maybe I'm biased by my non-orky viewpoint. Are there any ork players out there who *don't* consider lootas to be a pretty fantastic choice already? If you really want to boost them, how about a wargear option called, "Looted Gubbinz," or something that lets you choose or roll for some minor buff before the game starts. Like rerolling 1s to hit, boosting the
AP by 1, or adding 1 to their number of shots once per game?
Walkers: Why would they have an invulnerable save that a dreadknight has to use psychic energy to emulate, a dreadnaught can't get (outside of
SW), and a wraithknight's shield can't emulate? Letting them buy a personal-use
KFF as an upgrade would be cool, but an automatic 4++ would have to increase their costs by a fair bit. Especially on a walker that's already pretty beefy like an 'orkanaught.
Even then, I'm not sure I'd really want it on killakans. Isn't the whole point of kans that they're the cheapest melee walkers in the game? Maybe just let a big mek equipped with a
KFF hook up with them in his own custom kan suit? Or is that already a thing?
Other than that, I'd at least be willing to try out the changes you've proposed.