I don't see Jenova really being fightable for a very, very long time. Not because of its ties in Final Fantasy VII, that's easily rectified, actually, considering Jenova is, essentially, an alien who crash landed onto the planet, and it is very possible that the FFXIV universe's Jenova could very well crash land on Hydaelyn somewhere.

The biggest problem is that there are already two major dungeon arcs going on: The Binding Coil of Bahamut and The Crystal Tower. It'd be better for one or both to complete before going onto something else.

Another problem is that Jenova would need to be aided by someone with Hojo's level of biological/medical expertise, access to funding, and the will to turn Jenova into a weapon to use against Eorzea. That would mean involving someone from Garlemald that we've never seen before, because the only person even remotely close to that level of expertise that's still alive is Nael Van Darnus, but he's an engineer, not a biologist!

So... is it possible to happen within the game's lifespan? Yes. Will we be seeing this anytime soon? No way. Will Sephiroth be accompanying Jenova? Never in a million years.