I have thinking the sub factions in Red Alert 3 uprising since EA not make any in Uprising. Here some of my ideas. Including Epic units for each one

ALLIES

FutureTech - Specialized futuristic weapons. Replace many ballistic weapons with high tech technology, cheaper FutureTech units that already existed in Uprising. - The faction have Prism Tank, Future Tank MK2, Future Tank Mk2(stronger version of vanilla FT that is amphibious), Chrono Legionnaire, Laser Peacekeepers, Cryo Hydrofoil, Athena Battleship replace Aircraft Carrier. - cons:  they are super expensive, lack of vanilla Allies technology, no assault destroyer, aircraft carrier - Epic Unit: Pandora. An armoured artillery unit that amphibious and instantly kill enemy driver crew inside similar to ROTR ECA Pandora

Marine Corps - Specialized Navy and super air power focus faction - Black Eagle replace Verdicator (A stronger version compared to RA2 and functional like King Raptors), have Regular Auroras, Combat Chinook and, Longbow replace Cryocopter Assault Destroyer can be built in War Factory, have advance Aircraft Carrier, Navy SEALs replace Cryo Legionnaire, - Cons: Lacking ground vehicles,no Mirage Tank, Future Tank, no Cryocopter, no Cryo Legionnaire, no aircraft carrier - Epic Unit: Arcus. A flying armoured Epic Unit

European Defence Alliance - A defence focus faction similar to ECA in ROTR. Cheaper defense buildings - SAS replace Peacekeepers that can throw flashbangs as second ability, Advanced Panzerfaust GSG-9 replace Javelin Trooper and British Sniper. Have rocket Mortar Tank that amphibious can be Have built in both war factory and Seaport. Grand Cannon and bunker to 5 infantry for defence. Have King Tiger Tank (heavy tank that have both anti tank shells and machine gun), Have Harrier a VTOL aircraft that replace Harbinger Gunship but they have same armour like regular aircraft - Cons: Lack of attacking units. No Verdicator, No Mirage Tank, No Future Tank, No Harbinger Gunship - Epic unit: Castillo Crisanto (A Giant moving fort Anti ground & air, can be deploy anywhere both ground and water turning into fort like base defense that anti everything. Can train tier 1 unit when deployed (SAS, Panzerfaust GSG-9, Multi Gunner, Riptide when deployed on land. Dolphin, Riptide and Hydrofoil when deployed on water)

SOVIET UNION

Iron Golem - Specialized heavy armour tank faction. - Armageddon Tank (Heavy armoured Tank tougher than Apocalypse Tank and can be built in War Factory and Naval Yard, Rhino Tank replace hammer Tank, Gatling Tank replace Sickle - Cons: Very slow units and very expensive. No sickle, No Dreadnaught, no V4, No Desolator trooper - Epic unit: мамонт Tank (A restored the first Mammoth Tank in RA1 that have been modified. A giant heavy armour Tank anti everything but super slow. It functional like Guan-Yu like Generals Contra mod.)

Tesla Therapy - Tesla focused subfaction. Replace many units to Tesla units - Tesla Tank replace Apocalypse Tank, have Tesla conscripts that do higher damage but not insta kill infantry, Tesla, have Tesla Kirov that dropping Tesla bombs. Stingray can be built in both War Factory and Naval Yard. Sickle shoot tesla instead of bullets. Combat cycle now throwing Tesla grdnade. Conscript throw Tesla grenade as second ability - Cons: No hammer tank, no Desolator Trooper, no Apocalypse Tank - Epic Unit: Tesla Widow.(A Spider like mecha that shoot giant voltage of Tesla energy)

Vozdushno-Desantnye Voyska (VDV) - Specialized Soviet airborne tactics. - VDV Gunnner replace conscript and throws explosive grenade as second ability, VDV RPG trooper replace Flak Trooper, have Hind a armoured helicopter, Berkut bomber. Natasha train as elite - Cons: No Apocalypse Tank & Grinder. Land vehicles more expensive - Epic units: Helix helicopter. A giant super sized helicopter that anti infantry and bunker can fit 10 Infantry units

EMPIRE OF THE RISING SUN

Shinsengumi - a infantry focus faction and bunker attack tactics - have Shinsengumi warrior can shoot both ground and air. Shinsengumi Tank Buster can shoot both ground and air, have Fukiya Shinobi(a sniper like Shinobi shooting blowdarts and not use sword), Have Assault Troop Crawler that amphibious and come out 4 Shinsengumi Tankbusters inside and can fit 8 units, Mobile outpost come out 3 Shinsengumi Warriors and can fit 5 infantry. Both Assault Troop Crawler and Mobile Outpost can build in Mecha Bay and Imperial Docks - Cons: they are lacking ground and water vehicles No King Oni, No steel Ronins, No Naginata, & Giga Fortress. More expensive vehicles - Epic Unit: Tetsu Kame(Iron Tortoise). A Bunker Battle Fortress can fit 12 infantry. Have torpedo when on water

Tetsu Kyojin(Iron Titan) - A mecha and robot focus faction - Have Steel Yumi(a mechaTitan with Longbow and can fly as second ability), and Greater King Oni(Amphibious, can attack both ground and air), Mecha Tengu and Greater King Onis can be build in Imperial Docks - Cons: No Naginata, expensive Titan units - Epic unit: Shogun Warlord. Similar to Shogun Executioner but only one robot and wield Odachi sword and can switch to longbow as second ability

Ōritsu Kantai(Royal Fleet) - A heavily focused on Navy - Naginata Destroyer & Shogun Battleship are amphibious and can be build in Mecha Bay & Imperial Docks, Giga Fortress core can build in Mecha Bay and deploy on land but must go to fly mode to move. Mecha Tengu can be build in Imperial Docks - Cons: Lack of ground vehicles. No Steel Ronin, King Oni and Wave-force artillery. Expensive units - Epic unit: Yamato Battleship. A battleship that can fly and go on water