I don't see any theatrical Indy features sans Spielberg happening anytime in the foreseeable future (and I don't know that I'd want to), but if it absolutely had to and was going to happen, I'd like someone who's already part of the Lucas / Spielberg "circles," such as the aforementioned Bob Zemeckis and Joe Johnston. There's enough in their directorial styles that seems informed by Steve's own style that I think either could make a movie that meshes well with the other four.
Possibly better yet, how about Frank Darabont? He's done terrific work, and he's even worked on Indy already (and not just on his version of Indy IV). I don't think it's likely, particularly given his displeasure over how his involvement on the fourth movie turned out, but I'm talking more about who I'd want, if I absolutely had to pick someone other than Steven (besides, getting to not just write but direct might be just the sort of offer that could entice Frank back to work for George again - who knows?).
Though it was much different in tone and style from any of the movies than any of the movies are from each other, The Young Indiana Jones Chronicles is full of great storytelling, and Frank Darabont's contributions rank among the show's finest, IMO. Even if I'm not altogether sold on parts of his Indy IV script, I'd love for him to get a shot at Indy V - and what better way to entice Frank back to writing Indy again than by letting him direct as well? He's done great work in both capacities, after all...
(That said, I want to reiterate that Steven Spielberg really should be the one to direct all the Indy movies.)