Just as a thought experiment, would it be possible to create a bridge between "per encounter" abilities and "per day" abilities by using the current system of magic with slightly different spells?
Imagine, for example, a 1st level spell called Cantrip that has a duration of 1 hour/level and enables a caster to use any of the 0 level spells, 1/round and 1 at the same time only.
Or a Magic Missile spell that isn't insta-hit but requires a ranged touch attack, has a medium range, a duration of 1 hour/level and allows a caster to create 1 force missile and fire it off as a standard action, causing 1d4+1 points of damage on a successful ranged touch attack? Maybe add an option that allows him to discharge 1 hour worth of missiles (600 if my math doesn't fail me) to create a burst effect that causes 1d4+1 points of damage to every creature in a 30' radius?
Would those spells be overpowering as 1st level spells? Would it change the current gameplay drastically? The caster still has to manage a spell slot as resource, but gets longer use out of it. A bit like the old Melf's Minute Meteors, only with a longer duration (1 hour vs. 1 round/level).