The next step is finding the stages item, as that'll let me explore every level and map it out further. It could be absolutely anywhere though, and it'll probably take around an hour cumulatively to go through every level looking for it.
If I find it first, I wouldn't mind having the down upgrade item -- it'll make exploring these levels a lot easier.
Along the way, I'd like to map out the locations of bonus items, time items or magic items as I discover them.
Run 1
Went through worlds 0-5. Didn't find much, just a bonus item at 3-4.
Run 2
Encountered a game-breaking glitch and had to restart once I was 6 minutes in -- still haven't fixed a lot of the bugs I've noticed so might hit something like this again.
Went through worlds 6-9 and A-G total.
Found three bonus items, a time item and three items. The clock item at 9-5 will be particularly valuable while exploring.