Remember how Command & Conquer: Renegade was received when Westwood tried the opposite? Don't even think about it. There is no way to "improve" the gameplay.
I don't recall renegade, but "Starcraft: Ghost" was well-received in concept, even though the project ultimately failed.I think it could still be doable today with a rainbow-6 cross Assassin's Creed type game mechanic.
We already have a lot of book series which are spinoffs of or tied to game series, so I really see no reason one couldn't make an FPS-type game based on an RTS, or vice-versa. Blizzard's Overwatch could have had the same characters in something similar to LoL/DOTA.
Similarly, something like a new "Mass Effect" game could be great, so long as they have a good plot-line but to some extend honor their predecessors. ME2 changed how various game mechanics worked and did just fine. ME3 was good up to the end at which at lot of people feel it failed on plot
Nintendo has actually mixed up their core offerings a fair bit in some cases, be the the 2d->3d change with Metroid, or Zelda going from a 2d top-down to pseudo-3d, to full 3d, to 3d open-world. Being that BOTW is arguably one of the best games out in the last year or so, it's hard to say that you can stay within the say formula but still be "different" enough to be appealing.