The Tornado, and for a few reasons. I think it would fit within the 'timeframe' of the DCS series (it's a bit old now); It's primarily a ground attack fighter; There are squadrons operating or have operated around that region that is already mapped in DCS (i think, but don't hold me to that); Many nations have used the Tornado, which would make that installment far more marketable; And the final and most importable aspect, the multiplayer aspect in online videogaming of having a pilot and RIO in the same ship is going to be the best element of DCS . When this happens in the Apache AH 64A installment it will be a strongpoint that few sims have ever done successfully, and one that a few avaition geek buddies and I will be using till we grow old and the arthritis kicks in. The realism for the Tornado is obtainable, the technology isn't classified to such silly degrees as the more modern fighters, and who wouldn't love hoovering along at 500 miles an hour doing low-high-low ground attack runs...
And when was the last time a Tornado was a playable aircraft? Never. And if the F-16 is chosen (again) for a study sim, I'll still be happy to play it, but quite dissapointed that the developers wouldn't tread new and foreign ground to make something the community hasn't seen before... I know this is a 'what if' forum topic but hopefully someone out there is reading this and taking notes.
Vote 1: Tornado.
Also, for the Tornado fans out there... There is a great book called Tornado Down by John Peters & John Nichol, of their accounts of getting shot down in the 1st Gulf War. Awesome reading.