I would say developers should approach game design as players as well as developers. There are many things developers do that are not very good, some of them being idiotic.
Like in Majora's Mask 3D, Zora Link couldn't swim the same as in the original release unless you used his magic barrier thing. There was no reason to change it, yet they thought they should. The Twinmold fight turned into a wrestling match, which was beyond silly to me.
Dumbing things down is also something I despise. Doesn't make for good game design. Instead, they could just add difficulty levels. Would solve all the problems in that area if they did.
Of course, developers should make the games they want, but that doesn't mean they are doing what they should be doing, and it doesn't mean fans will like it either.
To some extent, I would say games should be made for the fans, like with Smash Bros characters that fans want should be included (within reason I suppose), or things like that.
Going back to your roots is one method to understand and improve future games in a series, or at least it's something that lots of fans would like, so it would sell to some extent at least.
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For Pokémon, I think difficulty levels could be added in as standard. That would help to please the more hardcore fans. Maybe more focus on telling a good story, and providing good post-game content as well.