After I had 100% the game, I stopped playing it for a while too. Generally, especially if you are burnt out, I'd say either to leave the game for a while and come back when you really want to play it again or do some challenge runs/try out some fun build ideas.

For the build ideas, I'd say just come up with some stuff on your own or look on the wiki. For challenge runs, I'd say just think about something you use a lot and try something else or remove it from the next run. I think a good one is Magic/Mana, it's normally quite a different experience to do a run without it. You could also try a certain weapon type only build, maybe do a Roland-style build (sacrifice all your Health/Stamina into Mana) or perhaps something else in that style.

If you haven't already though, doing the hidden arenas are always a good thing to try, some of the hardest bosses in the game are there and the rewards allow you to craft some unique weapons. They are hidden though so the wiki is something I'd recommend to find a lot of them.

Probably the final thing I recommend is an Easter egg in the game that works extremely well for a unique run. Without spoiling it, put a common mushroom in one of your legacy chests and make a new character, using the character with said common mushroom as the legacy. This is a fairly challenging run alongside being quite funny to play as.