The most important that comes to my mind is the Arthurian War.
From what I remember, a large chunk of it is inaccessible (or at the very least completely non-revisitable, one of if not THE only story places in the game like that).
It encompasses and wraps up a great deal of that storyline, so for it to be gone (or even if it's only not returnable) is a bit confusing. I could be wrong here but I remember there being a lot twisted up with this section of events.
I know you guys are against it but I'm with PD, not against old content returning at all. If anything, it'd make the game more fun as a long term player as it would give me more things to play with. I'd love for the entirety of it to be back in the game. I've amassed a fair amount of rare equipment and wouldn't mind in the least if it were available for all players, but I realize I'm just one perspective and that's a very non-AE policy.
That said, all past GEARS games returning would be a nice as others mentioned, there's a specific enemy that only showed up in the first one iirc (Sea Dragon) and they contribute a bit to the overall story as well.