The aesthetics in Returns and especially Tropical Freeze certainly lay the groundwork for a visually beautiful game. There were always plenty of areas with backgrounds begging to be explored.
For the sake of the studio, Retro should probably do something that appeals to the tastes of their original Metriod fanbase ... something more "T" rated. But if some studio were to pick up where they left off, or if Retro were to "Return" again, at some future point that would be incredible.
The main challenge, I guess, is that 3D platformers are have greatly fallen out of popularity, these days. Certainly the series would need to be updated in order to be successful with critics and sales.