A sustain mechanic could be interesting, and doesn't need to be limited to a single one; it could be any fixed number, or grow by level. However, it's IMO going to require a significant rebalance of the spells using it (at least unless you set the limit higher than will usually be hit, but then why bother).

If I was making a non-resource-depletion caster that's probably how I'd do it - in-combat casts just action economy, buffs having a sustain limit, utility stuff either counting against that sustain limit or a separate one.

Also could tie into magic items - everyone gets a sustain limit; non-casters don't inherently have a use for it, but anyone can take feats that give improved benefits (but use a sustain slot), and there are items that costs less than typical (but use a sustain slot).