There are a couple schemes floating around the OSR for spells without levels, or for having every spell have a 1st-level version and boosting its range, duration, and/or damage by using higher level spell slots when casting. That gave me a framework for some thoughts I've been mulling about making each spell an entire category or family, with the range of effects (rather than just numerical parameters) expanding according to what level of slot is used to cast it. A family of spells must be learned in order; you can't learn the 2nd-level spell until you know the 1st-level one, and so on. However, then you'd only have to memorize the base spell (1st level) in order to cast any spell in that family by expending a slot of the appropriate level.
As the post title says, this isn't a fully developed system yet, just some preliminary thoughts and ideas.
For instance, Feather Fall, Levitate, and Fly could form a neat little spell family. As a first level spell, it gives you a bit of lift, but not enough to completely overcome your own weight. You can't gain altitude, but you float gently to the ground, and you don't trigger most weight-activated traps. At second level, you gain the ability to rise or descend vertically. At third, you gain full three-dimensional movement.
Charms could be made to fit the pattern too, starting off allowing the caster to charm beings similar to himself (Charm Person), working up through normal animals, to monsters, to plants, to mass charms, and maybe even constructs and undead for the highest level versions of the spell.
Polymorph self? At first level, you can change to human or humanoid forms, up to plus or minus 50% of your size. You can't take the form of a specific individual, but you can alter your height, weight, body proportions, coloration, or apparent sex. Second level, partial polymorph - say, change your hands to claws, grow a tail, and things of that nature. Third level, you can change yourself into any normal animal of your level/HD or less. Fourth, polymorph into any monster form of no greater than twice your level/HD, as per the spell in the rule book. Fifth, polymorph self into plant forms. Sixth, polymorph yourself into mineral or non-living matter. Seventh, polymorph yourself into multiple creatures whose HD do not exceed twice your level/HD. Eighth, you get the special defenses of your new form, e.g. immunity to normal weapons, immunity to fire, etc. Ninth, you get all special attack forms too, just like the Shape Change spell.
Illusions work too. First level, you can cast an illusion that affects any one sense within a 20' cube. Second level, you can affect two senses; the illusion still needs to fit within a 20' cube, but you can move it around within the 240' spell range. Third level, you can affect three senses, and the illusion can fill the entire 240' range. Fourth level, four senses, and a static illusion can be made permanent. Fifth level, five senses, and you can program a sequence to go off under circumstances of your choosing.
I could go on, but you probably get the gist. Of course, some spells don't lend themselves easily to a scheme like this, and others won't stretch all the way up to 9th level. Is that a feature or a bug? I'm not sure. It seems like it might be a cool way to shake up the traditional spell system of D&D without warping the power curve too much.
Feel free to weigh in, or take the idea and run with it.