While I agree a Gate 3 will be needed sometime down the line, I think they really missed their golden opportunity to do it now when they are building around two iconic properties - Star Wars and Marvel. Both could've headlined a third park, but instead they now will have fully-fleshed out homes at DL and DCA.After this they'll have enough space to last them another 20 years in terms of new attractions built from the ground up. I imagine most of what we see in the coming 2 decades will be re-themes and land overhauls.
Of course there's always the potential of gate 3 at Toy Story or Angel Stadium if they need more land.