And, personally, I reckon they've been 'playing' with other planet types for a LONG time now. Even from the very start of developing Horizons, I'd be very surprised if they hadn't also been looking at how the engine worked with more complex planets like water worlds, earth-likes, ammonia worlds, lava worlds.....not that the results would necessarily be any good mind you, but I can't believe a dev or two wouldn't have been seeing what would happen.
I do think it will be a while before we see earth-likes, water worlds and ammonia worlds in particular, but not because they can't generate the terrain or water. I reckon the real challenge with such planets is the lifeforms. Within the bubble, it would make sense for some cross-pollination of lifeforms across, say, earth-likes.....humans being humans we like to have familiar animals etc around us, so I'd expect to see some imported lifeforms from other worlds in the bubble. But when one gets well out from the bubble, every single earth-like needs to be different - in terms of the lifeforms present, in terms of what stage of the evolutionary cycle they are etc, IF they intend to get it right. While one earth-like might be stuck at simple lifeforms, another might have had events that led to the development of complex lifeforms, and the next might be somewhere in the middle, and the next might have barely developed at all etc etc. It is this level of diversity....across the galaxy....that I think is the challenge, should FDEV deem it worth bothering to try to achieve. And that's on top of the diversity of life that would be required across each individual earth-like, let alone between earth-likes. It's already a little jarring, in my opinion, that the same bark mounds, for example seem to be so widespread - it's not just convergent evolution as the same barkmound, brain tree etc variants can be found all over the place (and it's the same for much of the life we've found so far in-game).