Um, okay?
I have no idea why you are fixating on 5e or 1e. Isn't the discussion at hand this proposed new system with all spells at-will?
Based on OPs post, this looks more like modifications to an existing system. I assumed 5e, and I doubt I was the only one.
To clarify,
@Redwizard007 is correct, for the most part.
My thought (for me anyway) is to take existing 5E spells and rework them to become at-will, thus removing spell slots. There are different things that might happen to achieve this:
1. All spells are nerfed to the level of cantrips. This means a lot of spells would really be removed as nerfing them to this power-level is not really effective. I mean how would you nerf
wish or
meteor swarm to that extend and still keep the spell concept or feel???
2. Spell power stays relatively the same, but since I don't want PCs fireballing every round, etc. the spells are reworked in any manner of methods (or in combination); such as increased casting time, costly spell components, etc. to keep them balanced.
3. New mechanics can be introduced such as a spell casting check, spell drain / fatigue, and so on as a balacing or limiting factor.
In one way or another, many posters have touched on these ideas (and probably others that I've overlooked as I don't have much time until tomorrow to do them justice).
Considering healing spells, for instance. If at-will, they could be used to "instantly" heal an entire party given multiple castings. Different ways of reworking a spell such as
cure wounds might be:
1. The spell only cures a single hit point with each casting. So, at most 10 hp could be healed per minute.
2. The spell might simply allow the creature to spend a Hit Die as if they had finished a short rest. So the limit is the creature's allotment of Hit Dice.
3. The casting time might become 10 minutes or more.
4. Holy water or some costly component, expended with the casting, could be added.
5. The caster must make a spell check to successfully cast the spell.
6. After casting the caster makes a drain check or suffers a level of exhaustion (or whatever).
7. The spell heals a target by draining hp from the caster.
and so on.