I don't expect this to actually be implemented because of the amount of work involved, but I also think it would be really cool it if did happen, so in the spirit of sharing farfetched dreams...
How cool would it be to have units and structures made of tech combined from multiple factions?
For instance, if you are UEF and an ally gives you or you capture a sera t2 engie, then if you select both the t2 sera engie and a t3 UEF engie, you could make a long range rapidfire ion gun (combination of ravager and sera laser/ion tech).
Or if you were to select both a UEF and cybran engie and build a combined tech factory, a t2 unit out of that factory might be a support unit that has both shield and stealth.
Ilshovahs with grenades and speed from Mongoose (factory built by sera + uef engies).
Percies with torpedoes from bricks (factory built by UEF + Cybran engies) or the percy gun on legs from a harbinger (factory built by UEF + aeon engies).
Sera could have RAS SACUs or other factions could have SACUs with OC (SACU factory built by sera+other faction engies).
Fatboy that walks like a spider and has a laser (built by t3 UEF + Cybran engies), maybe add lightning storm if built by sera engineer too.
Rapidfire mavor (built by t3 sera + UEF engies).
Again, all farfetched, probably units you wouldn't see built often, but could be awesome when they were built. We can dream, right?