Since missions are just text files, somebody could theoretically make a stand-alone program specifically for editing events and stuff. If one could make such an events editor from scratch, it would have all the expandability and features you could throw in there, and you wouldn't need to mess with FRED code at all.
Sure, I doubt that you'd be able to have FRED and the editor working on the same file at the same time, but if all you're doing is some hardcore SEXPing, then that would be fine.
(Please note: I have neither the ability or patience to do this myself, I'm just throwing ideas here)