I agree with Cooper's sentiments that a Sunflora / Carnivine / Cherrim evolution would be more like Prinplup -> Empoleon than Rhydon -> Rhyperior or Dusclops -> Dusknoir.
In the latter case, yes, I agree, the creative process would be hindered a bit. Rhyperior is literally just Rhydon+. However, with Prinplup, this Pokemon would not even be NU (whereas Rhydon was UU or even arguably BL in ADV) in the metagame. It is essentially an "incomplete" Pokemon, for whatever reason, be it stats, move pool or whatever.
Sunflora, Cherrim and Carnivine all fall in this category, I would think. Obviously, the UU metagame has not been fleshed out at all, so I can't claim that none would be used too often, but, from what I have played / experienced of the UU metagame, none of those are too common. I feel like all three are essentially unused. Looking at Shoddy statistics, they are 238, 239 and 259 respectively, all falling underneath Squirtle and Bulbasaur in terms of usage, lol.
By adding a type and thinking with the mindset to improve, we could definitely use a huge range of possibilities for evolving any of those three. Cherrim has potential as a supporter as it is, and I have already talked about how Fire / Grass is great typing for supporting. However, it is also a great typing for special offense, meaning we could use its original "guideline" to "build" into something more.
That said, I still support a new Pokemon, lol. I just wanted to clarify that evolving decidedly never used Pokemon does not really limit us, as they are basically "incomplete" Pokemon anyway in the competitive perspective.