Metamagic spells are an old idea. There was the three Extension spells in the first edition Player's Handbook and in second edition, the Tome of Magic had a lot: Alacrity, Augmentation I-III, Far Reaching I-III, Squaring the Circle, Sense Shifting, Dilation I-II, Divination Enhancement, Mordenkainen's Celerity, Magic Staff and Intensify Summoning. Each had its own rules on how long they lasted, how many spells they could affect and how powerful the spells that they enhanced could be. A simple question I asked myself recently is could there be variations that affect magic items or could the spells simply be used on magic items? In either case, there is a lot of potential for additional spells that enhance magic items if the GM wishes to explore the idea.

Spell components are also quite old and they could provide a backdoor into the idea of multiple school spells, something I worked on in the gaming content section (the 11 Labyrinth Lord spells). Expensive components, or foci using 3e terminology, could allow for spells that have more than one effect. Want one thing, use X component. But if you want another effect, then you have to use Y. This can be abused badly if the GM doesn't keep a firm hand on play. But it can also provide all kinds of nifty, inventive play. If a magic-user is out of a component but finds something similar in the dungeon or wilderness, what might the results of the casting might be? Personally I would reread Oops, Sorry in Dragon 163 for ideas, but the sky is truly the limit here for possibilities.