Huh...Davis certainly looks the part (perhaps a bit too pretty-boy, but not bad), but then so do a number of massess of clay and silicone over in the other forums. Does he have any more range than those do?
Kasdan has declined to take on writing assignments (at least from LFL), insisting that he's a director now, not a writer. Refused on the SW prequels, and refused on Indy4. Agreed it would be nice to see (although Kasdan apparently "advised" on Marion's dialogue in KotCS, so...maybe not so much).
As far as Indy 5, I think the next story should have Indy racing against his own country for control of a doomsday artifact. Amid the paranoia and nuclear fear of the 50s-60s there's a great story to be told where even the "good guys" maybe shouldn't have control over the ultimate power. Indy needs to keep anyone from gaining possession of the thing, and put it beyond reach. Some great storylelling opportunities as the "baddies" believe that they're acting in the best interests of the world as opposed to the usual cartoony supervillain "I vill create a race of atomic SUPERMEN!" shenanigans.
There's already a perfect artifact in the Indy universe for this, which even got briefly re-introduced in the last film. I can see it being a very cool full-circle thing to end the Indy movies if that artifact ends up back in the desert, maybe hidden in, oh, I don't know, Ethiopia.
'Course none of that will happen, but it would be nice to know what genre the three of them have agreed to this time around. Maybe Speilberg's haunted castle idea makes a return?