There is more than enough land right across World Drive for a big resort. Pretty easy to build a walking bridge over that big enough to make it a nice walk across to a backdoor/second entrance.
There is also enough land just southwest of the park that would have to be re-designated from not-suitable to build. Pretty much the entire WDW is built on a swamp and they know how to re-engineer the land to make it suitable to build, so I don't think that would be much of an issue. That would give them a back-entrance right into Streets of America/Lights Motors Action, or whatever would replace them in the future.
Even if it was a big issue (not suitable to build), there is a big tract of land between Osceola Parkway and Wide World of Sports that isn't far away in terms of distance.
If they wanted to, they could also put a tower at the southeast intersection of World Drive and Buena Vista Drive, with an entrance right onto Sunset Blvd in the park. There is already an entrance there (Cypress Drive) that could be used for the resort. You could probably fit 2 Bay Lake Towers in that spot.
Depending on the theme of the resort, it could mean a substantial uptick for the park. DHS could become a more male-centric park with Star Wars, CarsLand, and a potential Marvel resort. Again, I don't know if a Marvel resort is doable with the contract with Universal, but even if not, a Star Wars Resort would likely work just as well.