Yeah, you’re right! It didn’t occur to me that current space 4x games totally ignore gas giants except for anything than a resource. Realistically, correct, of course. But it was novel of MOO3 to include races that have evolved and survived in gas giants - which allowed for non-anthromorphised animals.
Endless Space 2 has an interstellar tree race which have their own form of space travel and colonising methods. I’ve not played them yet and so do not know how all that works. But in ES2’s galactic map, you can see their captured and colonised worlds connected with vine-like structures - suggesting that their mode of travel is different.
Also in ES2, space travel is not as limited as in Stellaris. It’s faster to travel through space lanes, but you can send your ships out to space (explored or unexplored), ignoring space lanes. Their travel time is the only penalty. That was the case with the two races that I played anyway. Other races may have other space travel limits and bonuses. For example, the tree race can extend their “vines” away from space lanes.
I’ll need to keep an eye on Stellaris’ (and ES2’s) systems. Both have limits I thought in regards to what sort of planets are available dependent on the type of star, I feel. I feel more so with ES2 because I’ve had that game longer and played it more.