Why not add some Mortars infantry , it isn't pratic ?
There will be variants for all factions, toxin or gaz mortar for GLA, burning mortar for China Flame, plasma mortar for the laser etc..
For some sides of GLA why not add some stolen aicrafts, like stolen Hind , stolen SU, or simply a kind of toxin bomber
In another part what about Blackbird for US and russian MiG-31 Firefox for China ?
Why do not equip some M16s and AKs with grenade launchers ?
If you add at least 1 or 2 units for some sides ingeniously , i dont think that it will make the unbalance
Why not add new faction like Russia or Europe, lol , if you dont find a place to put the new units ?
Adding same unit (even though with variant) would make each faction all the same. In Contra, each faction is unique. There are already a few types of artillery for each faction. Creator tried to create a unit that is going to be USED in actual battle.
GLA aircraft issue is discussed before. Currently as Arcadian Daze said, Toxin and Demo have aircraft. As a low tech faction, they don't need more (they're terrorists after all).
MiG-31 is used as AA for China and Blackbird is decommissioned long ago (since the end of cold war). 21st century USA uses spy drone and satellite instead of spy plane.
As for M16 or AK, Ranger has M16/M203 already (they can fire flashbang) but China doesn't have underslung grenade as weapon, so no grenade launcher (if you want to use one, there is a dedicated grenadier in Infantry Gen army at rank 3).
Creator also doesn't intend to add new faction.
PS: If you want to play mod with tons of units, I suggest Remix.