-Naughty Dog and SSM are kind of mysterious. SSM tends to do a lot of development support and ND has had instances where they've done development on two games simultaneously. They're big enough that they could surprise with a new game, but I'm going to say probably not.
ND are always working on 2 projects,.. they are just staggered. They will have started on whatever they intend to do next already but it will still be years out because they are almost certainly in pre-production phase. with that semi-mysterious stand along multiplayer they may even have 3 projects in the works.