I wrote this out for another thread but i feel like it could use its own
So in the field of design there's this concept called design space that basically describes the full breadth of every little thing you could possibly design within the given framework you have established. Pokemon has a fairly wide design space (as we can see with 720ish pokes and an estimated 40 megas pushing the number got 760ish plus alternate forms) but it doesnt have infinite design space. Game Freak and Nintendo arent stupid and they know they could make a generation packed with the things fans have wanted forever like fire/water type combo or a komodo dragon poke, but that would eat up all sorts of design space really fast thus making later gens harder to create. However realizing that design space is limited is not the same as running out of ideas. They are simply rationing ideas to make sure they have ideas to use for years to come and the franchise continues, something i and i'm sure many of you would like to see very much.

Its also important to note the difference between intuitive and unintuitive design. We all expect new type combos and really they aren't all that exciting from a design standpoint. I want to see a grass dragon as much as the next guy but thats not a very complex idea. Its intuitive, and that is the most important design to ration because it is satisfying to players. They "get it" and they have a positive gut reaction to it. Unintuitive design is exactly the opposite, ideas that change things up but arent all that obvious to the average player. Pokemon Amie is a great example if something that is an awesome feature but not the kind of thing anyone puts on a pokemon wish list.

So with both of these in mind you have to make sure that every gen takes up both intuitive and unintuitive design space. Its much easier to use intuitive design for obvious reasons and its also MUCH easier to run out of. Thus they ration it and fill in the gaps with the crazier weider stuff