Hello my suggestion are new bosses(mini-bosses) and new weapons!
For bosses : The bosses would spawn in the rare levels.
Beasts: The Blazing Vog-Cub.
Tactics:Similar to snarbolax but no bell is required to hit,when going up to ground it makes a fire(such as the ones from the oiler) and it deals fire damage and fire status effect.
Look: That vog cub in the beta(i think) but bigger, more fireish and beastly and nasty.
Slime: The Giant Green Jelly.
(Read wiki).
The Royal Lichen Colony:
Tactics:It starts big and it can divide it self into more little lichens and can make Spike shield around it(which can be broken) and will break eventually without being hit and in this time in the shield the lichen will heal.
Look:Bigger than the giant colony and taller and more spikes and a crown.
Construct: The Tortodrone
(Read wiki). It is the normal grassy torto.
Undead : The Great Soul of Owlite
Tactics:It will be invulnerable until you putt off the 4 blue candles with water blobs blue candles will act the reverse of yellow one and will lighten again within a few seconds,also it will spawn minion owlites,its attacks are shooting 5 bullets and doing the bite that kats do.
Thats it!
The weapon part is a lie