I would like an optional warning when I'm about to cast a spell that will end concentration on another spell; currently it's easy to miss in the UI that a spell requires concentration, or even that you're already concentrating, especially given how chaotic and complex battles can get.
This isn't a problem in actual D&D because if you mess up, or your DM reminds you you're already concentrating on something, you can simply change your mind or retcon as appropriate, but Baldur's Gate 3 is a far stricter DM and lets you make the mistake and completely waste a spell slot as a result. BG3 does also provide a lot of magic items that can cast concentration spells, so there's a lot more surface area for this to happen; even in regular D&D it's hard to remember all of your own spells, let alone an entire party of four's plus a bunch of magic items.
Simplest solution would be an optional (enabled by default for new players) confirmation box when casting a concentration spell and already concentrating, which would let the player confirm they want to cast the spell or cancel and do something else instead. It would also be nice if the UI highlighted concentration even more when a "conflict" like that occurs.