One thing I would have loved to see is 5.5E really try to fix the problem with Wizards. I think tossing fewer spells but with more freedom to do things like modify spells would have been a massive improvement, and I think
@Yaarel 's idea of Spell Points similar to the sorcerer is a great way to view things. Again, requires the sorcerer become something else, but giving it the control over all metamagic has always felt like them searching for an identity after losing the spontaneous spellcasting niche.
The short-rest spell points have the Warlock chassis in mind. Its spell slots are quantified, then spread out incrementally at each level. It works out to be 1 spell point per level + 1. The cost of a spell is its slot. (Thus a slot 2 Invisibility costs 2 spell points to cast. )
Because the refreshes are per short rest that can happen once or twice a day, there are fewer points necessary at any one time. This reduction prevents encounter-ending "novas", and forces a wiser use of spell points, which balances better with the at-will noncaster classes. Meanwhile a short rest is an hour, explains a refocusing after fatigue, and is friendly to most narratives about spellcasters. Noncaster classes can continue to press on without suddenly needing to sleep for the night because the casters novaed.
The removal of spell slots (except as a term for "spell level") deletes low-slot bloat. Higher slot spells can be prepped instead of the less usable lower slot spells. So there are fewer prepared spells that a player needs to keep track of during game play. (Note, the Wizard can still swap in new spells from the spellbook. It is only the the prep spells that are fewer.)
Because of the reduction of spellcasting due to short rest points, and the reduction of the number of prepared spells, there is actually more design space for class features. Compare how the Warlock has no empty levels (the two empty levels actually grant an invocation which is a significant class feature). The feat levels are at zero (origin feat), 4, 8, 12, and 16, and I would place the boon at 20, as an appealing chosen capstone, and to initiate epic leveling. The subclass levels are 3, 6, 10, and 14. The remaining levels are suitable for class features, each comparable to an invocation. The invocations can do almost anything: modify a spell, grant a special benefit, improve an earlier class features. This design space is suitable for metamagicking a particular spell, and so on. Compare how Warlock uses invocations to trick out the Eldritch Blast cantrip.
The main criticisms of the Wizard class are: it is underpowered at the lowest tier, has too much bloat, and its spells gain more narrative power across the higher tiers compared to noncaster classes. Reducing the number of prepared spells helps reduce the amount of narrative control, at least with regard to combat. The switch to a short rest rhythm keeps pace better with at-will classes. Finally, the remaining design space for class features for all fullcasters switching to spell points, can be utilized judiciously while making sure to balance with noncaster classes ay the highest tiers.
To delete spell slots and switch to spell points, solves so many design problems. The full caster classes become simpler to play, more narratively intuitive, and balance better with noncaster classes.
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