Was having a conversation about this with someone the other day. I think JK will get a reboot sooner, rather than later. I think JK4 is totally out of the question. Kyle Katarn/Dark Forces/reborn boss Knight is/was too big of an IP to be abandoned entirely.
I actually want a new JK, or a JK1 remake. I really love the SP story line. (Except JKA's story, which is sort of total garbage.) A new reborn boss Knight would at least give us a platform to make siege though, even if we had to do it through modding.
Yeah I doubt siege would be in from the start although I believe it was singled out for praise on JKAs release, even if they did try to revive the JK series I think the engine might not really accomodate siege as we know it.
They could make an objective style gameplay on battlefront but I doubt it would have the same variation of classes and tactics etc as a more jk styled game but I like Battlefront anyway so I'd definately get the game.