I find STAR WARS planets generally way more interesting than any planets in Mass Effect. Maybe it's simply because STAR WARS is a classic (or because it came first and Mass Effect came second, having to live up to it), or maybe, and more likely, it's because all you really see in all of the Mass Effect trilogy are space stations. We never really explored terrains, ever (and the boring and repetitive sidequests in ME1 do not count, they were all the same).
So on that note, I'd like to see planets like the ones we got on KotOR. Korriban was a little boring, but Manaan (even though it's also a "station", not a space station though) Tatooine and Kashyyyk were really cool (even though Tatooine was not really a novelty, since it's in the movies). Other than that, I think they should make these places beautiful (even if some of them are treacherous), and add a lot of effects, both from the planet's surface side (hurricanes, earthquakes, etc...) and from the galaxy's side (maybe we could witness a supernova, or other effects from space, while exploring a planet). They should also let us explore the Solar System in full. It was sad not being able to explore the mysterious red spot of Jupiter, the rings of Saturn, and all of our other neighbors in the original trilogy. Mars was there, but not quite. And then the Galaxy of Andromeda, which is probably where ME4 is heading to anyway. All of these would be interesting. And if they make us explore moons, please, don't make them as boring as Earth's moon, please! Make them like Yavin IV, or better.
They should also add interesting levels like KotOR had (and take the opportunity to add more effects to the Galaxy for us to witness during these missions, or the weather effects in planets), like when we were dressed in astronaut suits and walking around outside Malak's ship, or underwater in Manaan.