Hello,
First up, I want to say thank you for having created this awesome mod (and for reading this issue). Secondly, I have an idea on progression and new celestial bodies you could add for this mod.

=== Progression ===
You'd start with rudimentary rocket tech and could only go to Earth orbit/the Moon, but upon getting the materials for the Interplanetary Navigator, you could head to Mars. This is pretty much what is already in vanilla, but after that there isn't really much progression. Therefore, since you seem to be planning to add Mars' moons, maybe those two would have materials that would be needed to go past Mars, or some special ores, or something. Afterwards, you'd get access to the Asteroid Belt's largest body, the dwarf planet Ceres. There, you'd have to defeat a boss to get advanced heat shielding required for a trip to Mercury, and on Mercury there'd be metals that would speed up machinery and make it resistent enough to acids and pressure to let you go to Venus.

=== Celestial Bodies ==

• The Sun: you'd only be able to orbit near it, and you'd have to avoid getting too close or you'd burn up. But being near the Sun has its advantages: the Sun draws in objects with its gravity, and from time to time a meteor would spawn nearby that you could collect for resources.
• Phobos and Deimos: the moons of Mars. They'd have water in caves or craters (in a frozen state of course) and would have extremly low gravity, nearly non-existent. Maybe they could also have some mobs to make them more interesting. And, as mentioned, they'd have some sort of material (fuel maybe?) that would be used to go further into the Solar System.
• Ceres: as mentioned, it'd have a boss that you'd have to fight before getting advanced heat shielding required to go to Mercury and survive. But Ceres would also possess special materials, such as Salt in some craters, that could be used to lower water's freezing point. Ceres could have an underground ocean with fish or something, just to spice things up, and underwater there would be lots of valuable resources.
• Jupiter: perhaps, a bit of extra progression could be added past Venus, with a ruined civilization lying on Venus with crafting recepies for anti-gravity mechanisms and cloud solidifying mechanisms. This would let you "land" on Jupiter. Alternatively, players could go into Jupiter's orbit, as a starting point to reach the moons of said planet.

If you read through my rambling, then congrats :)

from time to time a meteor would spawn nearby that you could collect for resources

+1 to this idea, I've been looking for spacial exploration mods that would do this for so long
An addendum could be a meteor storm event that could happen on any planet (moreso on Moon, Mars and Mercury) from which you could protect with the use of some sort of machinery-shield, making the stay on alien planets more rewarding (and dangerous).

A couple of some other suggestions:

• Terraforming, make survival great again: upon achieving the stars everything could be possible (and resource-consuming), make the soil fertile, let the animals spawn. This could be enclosed to domes or to an area.
• Gates left around in the solar system: you could find one via astronomical searching on each and every planet, it would let you move between systems or galaxies, and would not be buildable by the player, only repaired.

Thanks for your comment^^ hopefully the mod dev will add some of these things