- Hide & disengage be changed to be actions (as per 5e rules)
- Cunning Action (level 2 rogue feature) to be reworked to include hide & disengage (as per 5e rules)
- Cantrips no longer causing surface effects in addition to causing damage and having their 5e damage values restored
- Monster HP and AC values closer to their actual MM stats
- Add more short rests (I'd start with 3 as a test)
- Remove healing from food
- Make Jump incur attack of opportunity, and remove the bonus action cost. It's just movement
- Add disengage, and let rogues use it as a bonus action
- Remove dis/advantage from elevation
- Give us a +2 AC when in an elevated position as a substitute for cover
- Remove advantage from backstab
- Remove ground effects from cantrips
- Change damage on cantrips to D&D rules
- Make "Help" not heal, only stabilize
- Fix the ward from abjuration wizard and add Shield spell.
- Give Abjuration wizard full ward after short and long rest as substitute for no ritual casting
- Remove about half of all special arrows and grenades from all mobs
- Rescale AC and HP more towards MM values, lower HP all around to make spells more impactful and more likely that we will come out of combat reasonably unscathed with good tactics and won't have to rest all the time.
Three more things I would add:
- Give Rogues the Expertise feature at 1st level like they are supposed to have.
- Add back in Thieves Tools (I don't like them folded into Sleight of Hand).
- Have races, classes, and backgrounds grant additional languages as appropriate (even if they don't actually
do anything at the moment).