Swords & Wizardry: Warlock Style
After playing the D&D 5th Edition Warlock class a bit, the magic system there seems like a good start as to how to go about things: Only a couple spell options during a crisis, but more ability to make other choices when things aren't going nuts.
This was what I had in mind while cobbling together the following hack:
Casters from standard classes obtain spells through the normal means (books or divine inspiration), and may learn spells of spell level equal to 1/2 their character level (rounded up).
However, keeping multiple spells prepared and "left hanging" at the same time, ready to cast at a moment's notice, is a precarious balancing act and demands a certain amount of constant attention to keep from screwing things up. As a result, spell casters can only retain one spell prepared at a time at first level, plus a number of additional spells equal to their level/3 (rounded down).
The time required to prepare a spell is 5 minutes per spell level. Wandering monsters and other time constraining events should be checked for every cumulative 10 minutes of spell preparation in a dungeon or other hazardous environment.
Preparation time for each spell is reduced by 1 minute for every two Ability score ranks above 10 (minimum 1 minute to prepare a spell). Low Ability scores similarly tack on extra minutes.
Spells can also be cast from a book (or clerical meditation) as rituals. A ritual is simply a spell which is cast immediately at the end of its preparation. Ritual casting does not require a spell slot.