Neverwinter Night's 2 style recharging.

[Sadrik]

Essentially, NWN uses the "spells/resources" per encounter instead of the per day paradigm (very similar to the Tome of Battle classes).

To adjust to this per encounter paradigm, we have to consider, that the standard system assumes four encounters per day, this reduce the spells to about 1/4th of the usual spells (round up... otherwise it'll be hard), and reduce hit points. Considering the fact that hit point lowering is far more dangerous... to 1/2?

Otherwise: Just reduce the CR of all creatures by -3/-4, and we'll get in similar range.

However, this will make combat far more deadly and... it would need very much playtest.

This seems about right to me.
Reducing the HP of everyone might be a bit much. I might try this equation though:
level 1:
Con score
level 2+
Hit die divided by 2 (round down) + con modifier [so wizard 2 and barbarian 6]

Also, a wizard maxes out at 4 spells per level a sorcerer 6 and a cleric/druid 5/4 but basically that winds up being 1 spell of every level if divided by 4.
This could be changed to a spells readied line rather than a by level spell chart. For instance: a 20th level wizard has 4 all the way across (spell levels 0-9) which divided by 4 would be a 1. So that is 1 first, 1 second, 1 third, etc. Now add up all of the spell levels and that is the spells that the wizard could have readied between each recharge. So, a 20th level wizard could have 45 spell level (+ int mod) readied. 5 9th or 45 1st would both be acceptable number of spells to ready between recharges. Once they cast those 45 spell levels they wont be able to cast anything until they recharge.

I would make it a 5 minute recharge not 5 second.

It does remove the reliance on the cleric which is pretty good (really good).