I suggest that, if it will not cause imbalance, make more subfaction-featured versions of all-subfaction-available units.
Example:
Pacific Front's Seaspear: A submarine version of Aegis, just like Akula is the sub version of Dreadnought. Seaspear has the same price/speed/attack power as Aegis. Since it has submerging ability, it has lower hitpoint compared with Aegis.
Russian Ignite Trooper: Russia use plate to armor their pyros, making them more strong and immuning to normal crush/dog attack. However they are slower and more expensive.
Chinese Moloch: A heavier version of Borillo, slower but stronger.
Chinese Seadragon: Another Seawolf, also slower but suited by heavy armor.
Scorpion Cell's Dunerider: I think he is better to be a Cell-only unit, with more firepower. PsiCorps already have Libra Clones (with detect ability) so they doesn't need Duneriders, while Headquarters will have a new amphibious infantry.
Headquarters' Dream Devouror: HQ transform their Dream Weavers. With metal shell, they can take more damage and immune to normal crush. The price is they are slower and a bit more expensive.
Headquarters' Reaver: A new amphibious infantry,cost $650, 140 hitpoint, 8 speed, plate armor, immune to normal crush and dog attack. It use plasma grenade to against ground unit/structures, 7 range.
Headquarters' Arrow Tank: A heavier Gatling Tank with medium armor, 310 hitpoint and 6 speed, cost $650, better assisting Colossus in anti air.
Also there can be: Last Bastion's Wolverene, replace Jackal. Same price, slower but suited by medium armor.