It sounds to me like most spells will need some augmented abilities, and I doubt they can be one-size-fits-all. In no universe is Weird a cantrip, for example. Maybe a level reduction, but that is both fairly flavorless and still has to be done on a case-by-case basis.
Maybe every spell should have at least one specialist ability, and every time a specialist learns an appropriate spell it learns one of these specialties as a matter of course. I'd keep the decision of which augmentation to learn at the character-build level so that spells that eventually accumulate many possible special effects don't impose the extra choice on every action, and so that adding new augmentations to the spell in a splat doesn't automatically make every specialist in that spell even more flexible. If a character learns multiple augmentations (say via a feat) then they'll have more flexibility in at-the-table usage.
For example, I could imagine a spell like minor image having the following special effects:
1) Use minor image as a cantrip
2) Whenever you expend a spell slot to cast an illusion, you also cast minor image.
3) Whenever you expend a spell slot to cast any spell, you can also expend minor image. For the next 5 minutes you may cast minor image whenever you cast a spell, and if desired the minor image may apply the illusion to modify the appearance/sound/etc. of the cast spell.
The first use is great for exploration and basic utility, the second use amps up pretty much every other illusion cast, and the third is great for tricky concealment and misdirection with other spells. For example, one could describe a fireball as a fiery zebra breathing across the battlefield, as an image of an apparently mundane exploding keg, or as a crystal sphere with a zillion lasers that reflect within the sphere many times in just a few moments.
If a PC eventually gained the ability to add another specialization agument to a spell they might choose both 1 and 3. Then they would always have the at-will use, but the option to trade it out at any moment for the alternate use.
It might also provide a route for "generalist" wizards, if those exist, to dabble in specialties very gently. For example, a generalist might take feat(s) to learn every specialization augment for magic missile and dimension door. Those are effectively signature spell for him, but it is still a far cry from the multitude of special augments a dedicated evoker or conjurer might gain.
Of course, I'm speaking as a person who really likes spell augmentations in general, and thinks something like delayed blast fireball should simply be a modification of fireball and not an independent spell. This cuts down on the number of the separate spells in the game (and mitigates the ever-expanding spell list of past editions), draws a strong tie between spells that share a fundamental story, and creates some useful structure for things like spell research. For example, perhaps classes generally choose to learn k new spells per level *or* 2k augmentations among all known spells. For some classes it may even make sense to trade some spells to learn a single specialization augment instead, since the specialization abilities will tend to be pretty good. Plus, I like the idea that developing a spell from scratch is generally a qualitatively different task to modifying a known one. In any case, that would give characters a chance to guide the sort of flexibility they gain.
It might also promote a bit more similarity between games where the wizard is drowning in found scrolls and one where a new scroll is a rare and awesome thing. In the latter game the wizard can sometimes feel a bit stunted by comparison. This kind of change to learning spells may allow wizards to function more consistently regardless of the scroll-finding environment, while still letting them have this iconic, but traditionally very campaign-dependent, ability for learning spells.