I would bet on F-18. When the next AC is done they should really start working on a dynamic campaign. It would make them ... complete.
While Darkwolf's post about that F-18c domain being purchased by ED pretty much seals the deal on what the "US fixed wing jet" will be, I think they'll already have their hands full modeling the F-18 to the same high standard demonstrated by BS and WH to present us with a half-decent dynamic campaign. Think about it: the F-18 breaks new ground in two major areas for ED: 1) air-to-ground radar modeling 2) carrier operations modeling (more specifically the logic for the marshal stack).
This is personal hearsay but it seems to me if ED figures they can't do something to a top-notch standard they won't do it. They've selected aircraft that don't have an air-to-ground radar and they have yet to do the dynamic campaign. I'd imagine these will require significant progressions to the game engine, or outright new modules for it (as in the case of the dynamic campaign).
If you recall the complexity of day-to-day carrier operations in Jane's F-18 you can see this will take some work to implement in the DCS engine; though the more I think about it many aspects of this have already been handled with the current air traffic control. If it really is the F-18 they'll want to nail down the AG radar which will take some doing.
I'm sure ED is more than capable of tackling carrier ops and AG radar, I just don't think it would be realistic to expect them to attempt a quality dynamic campaign in the same release as well. They'd be tackling three bears in one release - carrier ops being a black bear, AG radar a Grizzly, and dynamic campaign being a Polar (and HUGE one at that). I don't think they want to bite off more than they can chew at this point.
Again though that's only my opinion. Speculation does make for interesting conversation. . .