In playing 4e, one of the things that I miss about 3.x D&D is the feeling of a greater structure to magic. I would like 3.x magic more if it had more of a sense of mystery, but it's better than the total lack of structure presented in, say, a 4e wizard. I understand that Essentials at least superficially does something about this, but "I understand Essentials does X" is a common refrain for me since I haven't bought any of it yet (and given that I am not currently running a 4e game, I'm not likely to make another purchase soon). ShaggySpellsword has for a long time been my main source of information on all things Essentials D&D. Anyway.
As a further digression, I would like to sit down and kitbash the appearance of a system into 4e by tweaking existing spells, but any players of such an effort would have to be at peace with hand-typed character sheets, as DDI would regard them as mutant commie traitors. (Don't get me wrong, the original Character Builder saved vast amounts of time and energy. It also made kitbashing almost impossible, so we can be allies, but not friends.) Back to my original topic: 3.x structures of magic.
This is something I've messed with before, as displayed in my wiki. But everybody's done an elemental breakdown of magic, whether it's air-earth-fire-water(-spirit/void), earth-fire-metal-water-wood, or whatever. So, not so interesting anymore. I was flailing for something that felt different, but also structured enough to stop me from flying into a rage at squishy magic systems. Oh, and plains-swamps-mountains-forests-islands... also been done. Ahem. Though I like Birthright enough that I would appreciate a system that really drove home the connection between wizards and the land, that's not really where I'm going with this. I'm looking up, not down, and I have some ideas for constellations and planets.
My idea, which is still in a very rough form, is that each constellation might be a specific spell. The lines that "connect" the stars form its rune; different ways of drawing lines between the stars signify different spells. Sorcerers, therefore, imprint upon specific constellations, which they can change only with great difficulty. The planets in their courses pass through many of these constellations; wizards study these courses and derive power from them. They might focus on one planet, reducing their access to others (and possibly eliminating their access to an opposing planet, whatever that would come to mean). I haven't yet worked out anything about the themes that connect the spells of each planet, but I'd be taking as much inspiration as I could from astrology and various esoteric traditions. Stealing from Dragonlance the idea of one or more moons and their phases influencing spells in some other way is also possible.
I like for magic to feel like a part of the game the players can explore, and will want to explore. I want to be able to introduce new things they haven't seen before, but with some thought (or, in more obscure cases, research) can see how those things fit in with everything else. I want opponents to have governing themes with a bit more going on than "fire mage."
Fire mages everywhere hate me for this, so I'll explain. There's nothing wrong with fire mages, until PCs know that they're about to go fight a fire mage and bulk up on protection-from-fire effects. Then you have one of two situations, both of them not so hot. In one, the PCs' protections work, and the fire mage is toothless - he really should have learned another trick. In the other, the fire mage has fire spells that ignore resistance to fire, in whole or in part, so either he's doing some other flavor of damage (mixing, say, profane/necrotic damage with his fire damage) or he's casting FIRE rather than fire, and the PCs' efforts and resources to prepare were wasted. This situation generalizes to other kinds of preparations, but is at its worst with narrowly themed spellcasters or elemental creatures. Therefore, I'd have each planet draw on multiple concepts and be more than one-trick ponies.
Oh, another neat thing about this: spell scrolls look like starcharts, and that's awesome. Also, it gives a really strong reason for your setting to have a lot of circles of standing stones. If you're a darkness-beyond-the-veil-of-stars kind of person (though personally, I'm generally not) you're all set for something to start eating stars and destroying known spells.
If I ever get around to it, I'll work out some planetary themes and spell lists, and post them here. This will be perpetually low priority, though, what with all of the other stuff I should be writing instead.