Mario, Street Fighter, Legend of Zelda, Final Fantasy, Mega Man, and Tetris are all older series and are still going. Many not changing their formula any more significantly than Pokémon has.
As for the issue of introducing less new Pokémon- the motto "Gotta catch'em all!" hasn't been used since gen 2 or 3.
With over 600 'mons, I don't really think that's the main focus of the series anymore.
And therein lies my biggest worry- where exactly is the focus?
Multiplayer.
And if they want to keep the series going strong, they desperately need to expand upon that aspect. They've been taking some steps with the singleplayer by reinvigorating story elements (step in the right direction, but it still needs more), but I don't see much going on in multiplayer besides trading and battling.
It's been said before- but something akin to an MMO- with a big, open, dynamic and happening world where all the players can roam around and do ****- that'd be pretty cool.
In such a game, the Gym Leaders and Elite Four and Champion of different regions could be actual players who fought for it. Or they could be their own mini-guilds. Groups of people who share a favourite type, and new players can go 'round, challenge them- the usual- but with actual people rather than a one-shot A.I.