There is currently a suggestion thread that proposes placing the planets surface in another sector, one inaccessible except through the planet object in its own sector: whichbbehaves like a wormhole does and transports objects entering it to the sector the planet is physically in. The surface of the sphere-wormhole would be invisibly divided into hexagons/a dodecahedron and these tiles are mapped to the surface of the planet below. So youd "jump" to a planet by "entering close orbit/the atmosphere" and when youre ready to leave, fly up above a certain altitude, youre now in high orbit/the sector on the galaxy itself again.
It does mean planets end up instanced, but they can be as large as we want and accessible only through a transition to another sector like jumping, or flying between sectors does now (remember when flying throuh The planet terrain itself can be mapped to a bunch of hexagons (with normal cubic blocks) that represent the surface as it would appear in space, only loading those hexagons immediately adjacent to the player, and borders of hexagons at the edges warping the objects to the correct map tile on passing the border, treating it like a sphere in that sense. Not perfect still, but it would be more functional than the dodecahedrons we have at the moment in that it can scale to any size needed, and doesnt have weird transitional angles with terrain and gravity. Orbital bombardment could even be possible by letting projectiles travel through the wormhole, so a new projectile with identical properties appears out of the sky planet-side above the appropriate map tile at a random location.