I think the collective wisdom of the crowd has pointed out a few things... obviously any "third park" would be a huge undertaking, with additional parking requirements, the city of Anaheim being involved, presumably total redevelopment of a large parcel of land plus a new parking structure... it's a big deal and not something that would happen quickly.
Conversely, the area at the back of FL would be super easy to redevelop, it's even got access from the back for construction equipment, and you could build back there without disrupting most of the park.
Other than the planning phase -- which could take a long long time, given it's Disney, and they do sweat the details -- a couple of world class rides could go in there in a 18-24 month time frame. So, in the grand cosmic scheme of things, quickly. Probably they could start construction in 2014 or 2015 and open in 2016ish, if they had a vision for what types of rides they wanted to add and what kind of theming they thought would make sense.
TL would be medium term. It would require some effort and planning to figure out how to carve out that part of the park and leave Space Mountain able to be used during the construction. I would think that they would strongly consider taking out the theatre for EO and/or upgrading it to be able to have live entertainment like the Hyperion and/or upgrading it to be able to do something like the Muppets 3-D or It's Tough to be a Bug.
It's probably logical to do as much as possible all at once, given the disruption, so I'd think you'd see Autopia, Innovations, Nemo, the old dock area, and the People Mover all taken out. That's a LOT of space, probably 20% of the land in DL, and it could easily fit three or four new attractions.
So, between the design and logistics, it would be hard to even start construction any quicker than three to five years from now i.e. I'd be surprised if the first shovel happened any sooner than 2017. On the flip side, they'd probably do what they could to finish the project quickly with an 18-24 month schedule, even though it is more ambitious than the FL project.
Just some blather from reading these posts and Al's post... I am very excited about the concepts -- even a Marvel themed group of rides could be cool -- and I agree that while you don't want to plunk down a bunch of thrill rides with theming slapped on them, I think it's correct that they'll be looking to put some more exciting offerings in there aimed at the late teenage and college type crowd, also the older thrill seeker crowd.
The excitement is leavened though by the realization that it's really unlikely we'll be riding anything new until 2016 or 2017 at the earliest, and that's likely to be one new ride, possibly two, at the back of FL. Total redevelopment of Tomorrowland -- unless they've got very detailed plans from a while back that were put on the back burner -- is something we're not likely to see for close to a decade, in terms of actually being on a ride... and the third park is the kind of thing we might not see for fifteen years, realistically.
I'm not against reaching out to franchises for inspiration, but Marvel branding is completely linked with Six Flags. It would be a shame that Disney couldn't think of any concept more creative and original to place inside Tomorrowland than something that another park already does, even if 
