Well, then, let's play around with some ideas! The phase of divergent thinking (=brainstorming) was over pretty quickly, now we're engaging in convergent thinking (=narrowing it down to the option we have chosen). I could already open an In-Development thread for Lemmings Universe, but I want to wait with that at least until I've officially released Lemmings Open Air. Otherwise, it will look like I'm starting several things and never finishing any of them.
The next thing I / we should still decide on early is the tilesets that are going to be used. I don't want the chosen styles to be as arbitrary as in Lemmings World Tour. At the same time, I'm not ruling out tileset mixing completely. In Lemmings, Drugs, and Rock 'n Roll, I'm repeatedly mixing the same tilesets to emulate other tilesets that NeoLemmix doesn't have - specifically, those from Lemmings 3D. Now we could think about which tilesets to mix to emulate certain planets, and then whenever that combination of tilesets shows up again, you know where you are.
Let's start in our own solar system; this is going to be complex enough!
Mercury: L2 Space, orig Fire
Venus: nin10doadict's Lava tileset, orig Fire, namida / GigaLem Sky, ONML Rock
Earth: namida / GigaLem Tree, namida / GigaLem Desert, namida / GigaLem Metal, L2 Beach, orig Brick
The Moon: L2 Space, orig Crystal, ONML Rock, namida Mineshaft, namida / GigaLem Purple
Mars: namida Martian, orig Dirt, namida / GigaLem Desert, ONML Rock
Jupiter: namida / GigaLem Sky, namida / GigaLem Desert, namida Martian
Europa: ONML Snow, Raymanni Snow
Ganymede: ONML Snow, Raymanni Snow, orig Crystal, ONML Rock
Io: namida Honeycomb, namida / GigaLem Desert, orig Fire, ONML Rock
Saturn: namida / GigaLem Sky, namida / GigaLem Desert
Titan: ONML Rock, ONML Snow, ONML Bubble, namida / GigaLem Desert, Raymanni Snow
Uranus: namida / GigaLem Sky, ONML Snow, Raymanni Snow, Proxima Tile, Jarv Future
Neptune: same as Uranus
Pluto: namida Martian, ONML Snow, ONML Rock, Raymanni Snow
Here we already see the problem: A lot of the tilesets overlap. So visual recognizability suffers because the various planets and moons will look very similar, unless I only use specific, different pieces from the same tileset for two different planets / moons.
For example, the reason I listed the Desert tileset(s) for Saturn is that many of the pieces have the same shape as in the Sky tileset(s). Thus, I could use them as clouds, even inter-mix them with pieces from the Sky tileset(s), and thus, due to the different context, they would indeed look like clouds, not like heaps of sand.
Once we leave our own solar system, there are a couple of types of planets we could add that we don't have here. For example, a complete
water world, which could use Arty's Underwater tileset.
Most actual confirmed planets out there however don't have particularly memorable names - and we don't really know what they look like either, we can only imagine. In turn, if we fill some of those gaps with SciFi references (from Star Trek, Star Wars, or others), the world outside our own solar system might become more specific.
Star Trek is sometimes a little unimaginative when it comes to the physical makeup of other worlds - because most of the plots usually revolve around diplomacy and humanoid interactions, the vast majority of stories are simply settled on "M-class planets", which basically look like Earth, with just a couple tweaks here and there.
Star Wars, in contrast, frequently employs the idea of an entire planet having the same climate (e.g. Tatooine entirely a desert, Mustafar entirely volcanic, Hoth entirely frozen, Dagoba entirely a swamp etc.) That may be more versatile - but not necessarily realistic, thinking about how much climate zones differ just within the confines of our own planet. With the exception of a world that's entirely frozen (simply because it's too far away from its star), these "Mono Worlds" aren't all that likely to exist. But of course, they make for better levels / easier tileset choices.