So here is my 2 cents.
I went 50D to 5DII, and looking to 5DIII (soon, I hope).
You will be amazed at how much wider (and smaller) your pics will be on ff.
I have the 70-200 2.8L is II (great after AFMA), and the 400 5.6L (also great after AFMA), and 17-40 f4l (nice, at about f6.0 or so, but not great).
I would go with the 400 f4L 5.6. No is, but not a huge, heavy lens, so relatively easy to take along for the long shots.
There's also the 17-40, 16-35, and 14mm. Won't really be UWA on a 7d, but if you do end up going to FF later on it'll get you there. They will still all be wider than the 24-105.
Not sure I'd really recommend really going UWA, unless you're really just talking about being at the far edge of wide. Just rented the 14mm, and it's hard to use at anything other than right up close and personal, or back getting a view of everything (e.g. landscape or similar). So the 17-40 & 16-35 might be good for your 7d, it'll get you pretty darn wide even so, and if you go up to FF you will get down into UWA to wide.
Perhaps UWA is not exactly what I want.........Really, what I would like is a wider angle lens for trips like SF or New York where I would like to capture big buildings......where in the past I always fell short with the 28-135 that I used to have.
Also, times when the family comes over and we just want a group shot...I would like to be able to capture that type of shot.
Lastly, I am in Real Estate and it would be a plus to take pics of the interior without having to have issues with space limitations....but this would be just a bonus, I can always use a point and shoot for RE.