From what I've seen in modding primer, it's probably possible to implement extra planets as extra layers. This would mean
System layer, showing and allowing you to travel between celestial bodies within the system
Orbit layer for each celestial body within the system
Ground layer for each celestial body within the system (aside for gas giants)

The downside would be that you'd probably have to lock the mod behind Odyssey but the upside would be that you could take your gravship and explore the entire star system with it, not just the orbit. Maybe it doesn't have to be RimNauts 3 but certainly a mod like that for Odyssey would be cool