I played around with mixing up FS and Bf2142 vehicles and weapons early on when I was working on adding AI to First strike as a way of trying to track down bugs with FS. In fact, there are 2142 airships on several 2142 maps in the FS_SP and FSX mods.  Did not have any issues with vehicle or weapon compatibility.

Well I can't agree with this. I done my own tests on FS when modifing one 2142 map, and some vehicles for example eu_tank causing crash after his projectile hits anything, I had to change projectile's explosion effect to make it work. Another example is "mine_av_imp", I used it against eu_tank, no effect - tank full hp. Anyways I don't know how you tested that, but fact is some weapons/vehicles need fix in order to make FS vs 2142, also I didn't said all weapons/vehicles don't work. Maybe I should mention too that not all EU huds working?

Yes, it's much easier to make flags for Imperials and Rebels than make holograms for EU and PAC, but who cares about it when it's gonna be only "additional map", or maybe you think about something more, then we can care.

However, there is one issue that I can thing of with a mod that pits BF2142 factions vs FS:  What to do about the CPs?    BF2142 uses flags, but FS uses holograms.   You have to choose one or the other.   There are no EU/PAC CP holograms and neither are there any Imperial or Rebel flags.   It would be easier to create flags for Imperial and Rebels than it would be to create the Holo CPs for EU/PAC.

My idea is simple, Battlefield 2142 vehicles vs First strike vehicles map or something like this.
I played a little with Talon, RsfAngelo and TankCrew on my server, I spawned few gunships and we fought 2142 vs FS . It was really nice fight, expecially gunship vs xwing