How overpowered would a staff that lets druids cast wizard spells be? Example: it has 3 charges per day that let you use druid spell slot to cast ANY wizard spells of 2 or 3 levels lower. So a druid could use a 3rd level druid slot plus one of the 5 staff charges to cast magic missile, for example. This would be some insane versatility and I considered making it a preset list or something. I want this to be a legendary item that comes in 2 helical staffs that intertwine so it won't be easy to find, but I also don't want to make the guy a straight up gestalt caster. Any suggestions for making this concept somewhat balanced?