IMO they need to give the franchise a rest. I love the uncharted games but I've had enough for years now. Felt the same way about Gears of War but MS just keeps churning them out, same with Halo.
When they do bring it back. Do something new, change it up. Similar to how God of War came back but changed a great deal from the original games.
Eh, it's already been 2 years since the last Uncharted, and if Sony did decide to open a new first party studio to take over the Uncharted IP next gen, new studios take time to build and staff up, plus the time required to develop a AAA game, so we'd probably be looking at a 6 or 7 year gap between Lost Legacy and the next new Uncharted. That's a plenty long break imo. I agree about doing something new though, I would like to see a new Uncharted series with Cassie Drake as the playable character, and mostly new characters, aside from some appearances by the older characters. Some gameplay innovations would be nice, more open maps as well.