WWI would be fun, if there were some point to it objective wise. I've thought personally of adding a Zeppelin element to the game, having hangars that spawn a "CV" type unit, that doesn't launch aircraft, but has manned positions for fire, and is capable of carrying ord to a strat. I'm not sure if bombing should be a position, or an automated thing once over a Town, but basically the unit can be piloted, but is little more than a gunship until it reaches a bombable target, at which point it does it's business.
This can be negated by bombing the Zep Factories, or maybe smaller supply depots with light bombers, or the zep can be intercepted (much like Red Baron) en route to target. It's not a huge objective, but it gives something to do in the air, that doesn't necessitate a ground battle/base capture element that wasn't really present in WWI.
Tanking, I'm sorry, will be worthless. Historically, they were nothing more than a moving barricade and trench buster. But if we want to add some element of playability to the WWI arena, there needs to be some sort of "Winning" element to it, or it's just a DA for people who don't like flying free tempests and using F3 mode.
As for Korea, I'm all for it, and I don't see how it goes against the game at all. It's the last gentlemanly conflict in the skies, where there were no heat seeking missiles and BVR shoot downs. It was down dirty, personal, and utilized much of what is already in the game. Adding a few Jets and Bearcats won't hurt anything.