Here is a few rules changes I made to Basic fantasy to get the feel I wanted for my lost realms game.
1. All characters start with hit points equal to their Con score plus their level. Fighters and Clerics gain a 5hp bonus.
At levels 5,10,15, & 20 fighters and clerics get a +2 hit point bonus.
2. Spells that scale. Spells like fireball that normally scale a die per level now scale a die + 1 per level of the caster.
3. Spell focus. A spell user can Choose a spell to cast, but instead choose to focus the spell delaying its effect. For every turn the spell is delayed add +1 to the damage and difficulty to save against when the spell does go off. If the caster is attacked or otherwise distracted during a focus they must make an Int save to maintain focus.
4. All characters can choose to make a defensive action. Adding +1 to their AC until the beginning of the characters next turn.
5. Recovery roll. After a combat any character who receives triage, Small breather, stop bleeding, bandage wounds gains one hit die in hit points.