So I drafted out an infantry class mod on paper, but your mod is so close right now I'm not sure it is worth the effort (and there might be some animation challenges that are beyond my abilities). Anyway, here is my infantry mod draft for you or anybody else to poach or use:

New Class Development Rules:
1. An optimal class is one that brings a new strategic layer to the game ->
The infantry class can spread reduced damage across two shots with their assualt rifle (not their pistol!) as a squaddie skill (other classes can't do this until at least sergeant).

2. Is balanced against other classes ->
This class as described below is more diverse than gunslinger and able to do comparable (but slightly higher) damage output (which is fair given no squadsight).

3. Is balanced at each rank ->
Each ability at a given rank in the "assault Infantry" path or "support infantry" path is comparable in power.

Proposed Abilities:

Squaddie Ability: "Semi-Auto" - Reduced action cost (1AP), Damage (-1dmg), & ammo (no ammo cost if 1st action). This ability would get its own shoot icon like the pistol fire icon and would optimally have a slightly different animation with a single shot fired.


1. = Assault Infantry path
2. = Support Infantry path

Corporal:
1. "Hardened" - +10 defense (just like the mutons get).
2. "Smokeout" - Smoke grenade only costs 1AP (this mod would also need to include the script to actually make smoke grenades work (thanks fireaxis), and I would probably crank up the defense bonus on it to 25%. (There is already a mod that does this that could be incorporated.))

Sergeant:
1. "Rifle Deadeye" - Sharpshooter ability import for +2 dmg on a semi-auto shot (but aim modifier reduced from vanilla to 0.15f which causes an avg aim reduction of ~10-15%.) Can only be used on 1st semi-auto shot.
2. Covering Fire - Specialist import.

Lieutenant:
1. "Lightning Hands" Pistol - The one pistol ability listed anywhere here for a small bonus 3rd shot. (I considered semi-auto 3rd shot but I want to keep it balanced with gunslinger dmg and with the other ability choice for this rank.)
2. "Opportunist" - Eliminates aim and crit penalties on overwatch.

Captain:
1. "Rifle Face-off" - Adapted sharpshooter import to fire a semi-auto shot at every enemy in LOS.
2. "Kill Zone" - Sharpshooter import adapted for assault rifle (all these overwatch abilities use full-auto (regular shot) mode.)

Major:
1. "Implacable"
2. "Ever Vigilant"

Colonel:
1. "Rifle FanFire" - Sharpshooter import adapted for rifle. Shoots 3 semi-auto shots in a row at one target.
2. "Deadly Guardian" - 100% chance to perform additional overwatch shot if previous overwatch shot hits.