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Next most possible race is obiviously an Imperial Guard (opinion as a player)
We have most of weapons done for them
and i have in mind some suggestions on classes and gameplay assets

NOTE , imperial guard gaining points x1.5 times faster then marines or tyranids
Why? guardsmen obiviously got lower health then tyranids or marines , die faster and cant be usefull in close combat , so they suppose to reach higher tiers faster.
NOTE , all guardsmen got about 75-80 hp , im not sure about the number , but seems about right , due to the game balance

most possible to create , even with currect stuff : +
needs work or alot of work :  -

Tier 1
-Guardsmen with Lasgun +

• no recoil
• ignores armor
• low damage
• average rof
• ALT attack with baynet

-Guardsmen with Autogun +

• small recoil
• doesn't ignore armor
• higher damage then lasgun
• high rof
• ALT changes fire modes , single and auto. Single shoots more accurate then auto

-Sergant with combi CSword and Bolt/Laser pistol +
Tier 2
-Veteran with Bolter +
-Kasrkin with Melta +
-Kasrkin with hellgun -/+

• shoots faster then lasgun

-Commisar with powersword and plasma pistol  -
Tier 3
-Kasrkin with grenade launcher  -
-Guardsmen with deployable heavy bolter (and lasgun)  -

• 1 heavy bolter per guardsmen
• destroyable
• can be used by other guardsmen(in case of death of owner)
• heavy bolter dies if player choosed different class
• heavy bolter dies if player deployed another heavy bolter
• heavy bolter can be picked up by same player and redeployed with same health it has
• heavy bolter have bigger clip then the regular one

Hypothetical
I like your 1st tier suggestions, but I think lasguns shouldn't penetrate armour, but should have unlimited ammo instead (no reload times, no dying just to get a new clip).

For the second tier I would suggest 3 of...

• Gurdsman with special weapon (metla/plama/flamer/grenade launcher)
• Ratling Sniper
• Orgryn Trooper (early "tank" class)
• Commissar (I don't really see the point, since FPS players don't have the same psychology as IG)
• Sanctioned psykyer (psi attack ignores armor)
• Sister of battle (why would a SOB be higher level than a tac marine?)

For the 3rd Tier

• Imperial Assassin (various)
• Sister of battle with Heavy weapon (ML or HB)
• Sentinel

Practical
Even at increased points gain balancing warriors to guardsman is going to be a hell of a stretch.

We've got world models for some of the 1st tier stuff, but no view models or suitable hands.

I always viewed the teams of EX as Humans VS Tyranids, with all forms of imperial soldiery on the Human team.  Adding a tier 0 with IG on the human team  VS Gaunts on nid team would be more practical.

IG are not a terribly popular race, I get asked when Orks and Eldar are going to be added far more often.

I would be easier/faster to convert the UM assets to CSM than create all new assets for IG.

well , why did i suggested to have more point gain for the IG is simply to do the favorite tactic of IG , meet with the enemy as fast as you can and die in the name of Emperor
after that you'll meet enemy with more powerfull weapon like melta and will be able to fight back
and god help enemy if IG manage to fight back well , because it will be tier 3 for IG next time they die.

as for tiers
why no Kasrkins? they are one men who gave good name to IG
and they usually carring more powerfull guns

as for the game of FPS with 2 sides (i mean a practical side of gameplay here), one of which is meele (atm), guns means more then health

-ratling snipers , are they in IG? i mean , heard of them , but never saw them as a legit unit anywhere
-Orgryn ...i guess its alright for t2 unit , but that brings me to the memory of huge characters spam
-Sanctioned psykyer , now that souns like tier 3 actually
-Sister of battle? why?
-Commisar could be able to frindly fire player with health lower then 15 and buff boom ppl around with higher ROF for several seconds

-Imperial Assasin , sounds really legit , invisible , hight distance attack , good damage
-Sisters of battle again , now i see your approach here , you dont want to jerk around with stationary heavy bolter code , understandable actually
-Sentinel....im gonna disagree with you , since for now , mod is infantry based and i cant see a sentinel in game anyhow

Besides , Stahl is a fan of IG , so he would gladly move the IG to the sunlight
Might be CSM vs IG in some future? or possibly even to choose teams , which is about to fight

without tanks , ig is useless. theres just no way to balance them right and make them feel true to lore... on some maps though ig vs orks or stuff like that would be much more doable

I missed Kasrkins off my list mainly because i'm an oldskool IG gamer and they just didn't occur on my mental army list.  I've got nothing against them, but why play a guardsman that's slightly better all round, when you can have fun with a brutish Orgryn or a Snipy Ratiing?  I'm concerned they might get overlooked for more distinct, sexier classes.

I've got a couple of squads of Ratlings in my old (3rd ed) IG army, I should be very sad if my little abhuman snipers are no longer codex legal.

Orgryns wouldn't be anywhere as big as most of the Tyranid classes, so I wouldn't worry about the size of them.  Similarly with the Sentinel, just think of it as infantry the size of thrope.

Sanctioned Psykers would be a very vulnerable class, which is why they would be tier 2 not 3, but give the IG a chance to counter a fex if they both hit 2nd tier at roughly the same time.

SoB because I want to have a single player operating a heavy weapon in tier 3, AFAIK SoB are the only imperial unit that does that, but I don't want her to be the odd one out.  So basic SoB at tier 2 and Uber SoB at tier 3.

Commissar... seems very forced, I'd rather just give a commissar model to EX heros who choose the melee IG wargear and get a genuine leadership boost.

I would really rather prefer to pit a new 3rd race against either of the exiting ones.  Could I persuade you to back chaos undivided renegade IG as a 3rd race?

re tanks: Possibility of a Vox operator class at 2nd or 3rd tier, with the ability to spawn players localy (grav chutes) and call in artillery strikes, but then we'd have to limit them to 100% outdoor maps.