(1) I think there's a case for a Disney World stop if Disney is inclined to cooperate (which might involve some sort of "sidebar" off of I-4; frankly, a good candidate would be roughly where the Casting Building is or the gas station next door...it's not at one of the parks [1] but it's within the general network of the Disney bus system.
(2) Going via SR-528/I-4 also includes the not-insignificant advantage of being able to potentially add an OCCC/Sea World stop (at least for some trains). I don't see this as a short-term item, but I could see it as a longer-term thing. TBH this looks like a better option than running a streetcar/light rail line to the airport (or probably than running Sunrail to the airport on its own...in the end it might just be cheaper and better to build some sort of track interchange and let Brightline dictate frequencies).
(3) Lakeland is probably an incidental concern. It's not the smallest city in the region, but playing all of those games to move the line from two miles outside of downtown (probably with much better TOD options; you've got a lot of open space out by I-4, so there's room to really grow a development in some places and other places you could probably acquire and repurpose some neighboring land...and looking at an overhead shot of the I-4/US-98 interchange, the interstate's median widens substantially, almost like it was...I dunno, planned to have a station there?) to downtown with limited TOD.
(4) Ok, Tampa. You have a wide preserved median on I-4 that could easily fit a station out by Ybor City. Same thing west of downtown. There's a mile or three of area between these stretches on I-4 and I-275, however, that doesn't look like it has a decent ROW. Also of note is the fact that the Marion Transit Center is here.
Looking at Tampa Union Station, /if I wanted to use it/ I'd stay on I-4 until Nuccio Parkway/12th Avenue (conveniently about where the wide median ends) and then cut over that way. You're probably looking at about a 1500-2500' connection there. The problem is getting /past/ Union Station, since the tracks run down the middle of Polk Street and you have both one overhead footbridge and a parking garage stradding the tracks to deal with. Between that and the fact that those tracks are CSX's access to Port Tampa, I think you have a very real problem here...but at the same time, the I-4/I-275 interchange might be enough of a mess that this is more feasible to work with. I guess the other question is whether, if this were happening regardless, Tampa/Hillsborough County would look to piggyback a commuter rail operation on this (a la Tri-Rail)? If they're interested (much as if Jacksonville is interested in something towards St. Augustine), that might pick up some of the tab.
Ideally, I think Brightline would want two stations in Tampa (one somewhere vaguely near downtown and one at the airport) since I see the airport as sort-of an afterthought here. I think stub-ending near Ybor City might also be a viable short-term solution, too, since dealing with that chunk of freeway mess and/or downtown is a nightmare (and a stupidly expensive one at that) in my mind. I just don't see any particularly good options for dealing with that.
Finally, I think Jacksonville will come first. It is cheaper, they already own the whole ROW, etc. There's more ridership to be had on the Tampa line, but there's also a lot more work and expense involved.
[1] I actually think a station at one of the parks is a questionable idea for various reasons, not least of which is that grade crossing in the Epcot idea. However, I also think this comes back to "How will Disney interact with us?" insofar as if WDW were willing to commit to cutting most or all of the DMX buses it might help take some sting out of any messing with the routing.
