Partially the market is stagnant because the big vendors will not try anything to REALLY stand out. (Well, I'll admit foldables are something new, but other than that.)
Keyboard phones? When TCL make the BlackBerry KeyOne, then the Key2 came out, they sold as fast as they could make them. They said they didn't hit the sales they wanted, but they had people wait weeks, find out the new batch sold out in like a day, then say "the hell with it" and buy something else. I love my KeyOne and am pleased that at least I can order some keyboard phone from UK and pop my SIM into it if my KeyOne wears out (I LOVE having a keyboard to tap things out on!) I'm sure the likes of Samsung or LG or the like could manage to produce the phones fast enough to not have people be turned away due to slow production.
Sony made those phones with a Dpad on it and you could play Playstation games on it. I don't think you'd want to try to play like PS4/PS5 games on the phone (if a phone had enough power, it'd probably run pretty hot!!!) but if you had something that'd play Switch games or the like it'd be pretty unique and likely be a seller.
Just making a phone where the battery & screen are easily replaceable would make a phone stand out compared to the rest -- of course the phone companies would be nervous that if someone's phone is too repairable they won't buy a new one. Most people still find their phone is getting "long in the tooth" after 5 years or something anyway, I'm sure people would still replace the phone even if they don't have a cracked screen and dead battery pushing them into it.
The vendors all go for making a slab, as big a screen as they can fit, as thin as possible, so they end up making what appear to all be the same phone, then wonder why their phones won't stand out in the market....