- Familiars shouldn't be able to attack unless you are a Pact of the Chain warlock, even then you have to give up your own Attack.
- Familiars should be able to grant the help action and deliver touch based spells with their reaction though.
- Reactions need a rework it's currently impossible to use featherfall for it's intended purpose. It's also impossible to implement the Ready action
- You shouldn't be able to tell your percentage chance to hit, you should just see your own result and whether that hits or misses. Same with knowing the DC of skill checks.
- You shouldn't get advantage for having high ground. If anything high ground could allow ignoring half or quarter cover if you get a better angle.
- Disengage should be separate from jumping and an action
- Jumping should use 5e jumping rules. Long jump is your strength score in feet *if you have a 10ft run up* otherwise it's half that. High jump is 3 + strength mod in feet with the 10 run up. You shouldn't be able to jump onto the roof of a house without the jump spell or the boots of striding and springing. Or be a monk. It shouldn't be a bonus action, just part of movement.
- Fast travel shouldn't use teleportation unless it's somehow tied to the wriggly wriggly worm in your head. That's pretty high level magic that you just use to save walking 100m. If fast travel is needed, just have it be signposts and there's a time skip while you walk there.
- Weapon skills like cleave and pin down aren't 5e things. If you insist on including them, make them part of a new feat or a fighting style
- It's too easy to camp, and there doesn't seem to be any chance to getting attacked in the night as you seem to be in some demiplane. Watch orders and random encounters are part of 5e, unless you have tiny hut or magnificent mansion.
- Short rests should also take an hour, and give you the option to spend hit dice to regain health. Not sure what the method is of regaining health on a short rest currently is. Depending on where you decide to short rest there could be interruptions as well.

I'm sure I'll think of more later.