I would love the addition of a simple practice range, where the player has full control of the game : access to all items, spells, talents, and able to spawn any enemies, bosses etc etc I am sure some people like myself would love to experiment in such a gamemode with various builds and item synergies, also learn abilities and possible synergies and the details one can miss in a normal play through. Not the most groundbreaking feature but would be a neat thing to have access to
Another idea inspired by this practice range is a new gamemode, being a horde mode : surviving on a large level for as long as possible, with waves of enemies rising in difficulty, access to merchant and librarian, and a higher level cap, and boss waves and bonus waves of ridiculous amounts of surround types enemies like bats rats slimes etc, with a yendor cameo as a mega boss wave, could be a fun idea, with lots of customization/challenges possible, can even go hand in hand with the new teased ranked mode.
As always I love the game and can't wait for early access, I hope these ideas can give you some inspiration lol