No questions about mechanics, huh? Well, have a load of speculation on Primordial Moon's skills shop instead.
Mistress of the Tide is the next Super ability we can see for Moon alone. Expensive, at 700 exp, it also has a requirement we don't know yet. My guess is that it requires Radiant Conductor, due to this line in the flavor text: 'Become the conductor of the battlefield.' I think this results in a battlefield wide effect, probably altering damage of all parties present, potentially increasing one side and decreasing the other side. Might also act like the 'tides', having an increasing then decreasing effect over time.
Radiant Consuctor is probably good even if it doesn't help unlock Mistress of the Tide, as it should allow us to either increase or decrease Sun's damage output - a useful skill. This may enable us to fire warning shots that don't kill one of the people we're trying to warn. That it also bolsters Moon's comparatively anemic damage output can only be a good thing.
Otherworldly Radiance probably 'just' alters Moon's Resilience, adjusting the rate at which it increases, making us substantially different tankier and increasing the effectiveness of buying levels in Veil of the Lunar Goddess. Definitely buy this before Shining Protector.
Shining Protector 'allows Eva to protect others in battle.' This probably means she takes hits instead of her allies - or perhaps enemies she wishes weren't enemies. I'm sure we'll get a number of uses out of it, in particular for protecting Sun. We almost certainly want Otherworldly Radiance first.
Neverending Moonlight provides a 'special' Omni-caster. Not the regular one attached to a spell, a special one. Hypothetically allowing us to multi-cast multiple spells at once, instead of the same spell multiple times. Damn useful, even more useful when Moon figures out a few more spells - ooh, maybe we can get a barrier spell! 'Shell of Moonlight' or 'Radiant Lunar Barrier' or something, protecting others with our Light.
Hmm, what else... eh, I have no idea what may yet become available. At the time of this post, at any rate. I'll do Sun next. Sometime. Might even be today, too!
As for spending exp, I can see several paths - not sure which to take. Tank, Battlefield Conductor, or super-caster are all up next, and I don't know which we want most - or we can work on Moon's weapon and healing spell. It would be nice to have a heal that doesn't take 25+ casts to heal Moon to full.