Hopefully a new IP. Uncharted is wrapped up, they could have left it alone at 3 TBH. Let it end on the positive note it did.
If they make a LoU3, they need to make an all new cast where we see other events in the same world. Joel and Ellie's stories were done with the first game, so they shouldn't have tried to drag it out. That world has a lot of potential, but it is time to move on from the Ellie IMO.