I have this category up for Speedrunner's Edition currently, but wanted to mention it here to see if anyone would be interested in making it a category extension for vanilla SMB3 as well. The goal is to complete the game while finding and activating all the giant question blocks in the game (there are 18 total). Warp whistles are allowed, but no wrong warps.
Here is a rough guide of the route I put together: https://pastebin.com/CfuyQr3D And here is the guide I added to SE showing how to get to every giant question block: https://docs.google.com/document/d/1m84_mj0OpU4FRpiZKST5OeNKPQZGKTHbKgnQOLbb1Qw/
I think it's a fun and unique category that isn't too elaborate with secrets, but still allows you to branch out and see some of the extra areas you don't see in other categories. Definitely give it a try if you want both the speedrunning experience and the thrill of finding secrets you get from casual play.