Mario's New Islands - by Zone Z5 - Hacks

This would be a very nice hack if there wasn't that (mentioned by you) OW oddity. The levels were fine actually, and the OW error isn't that big at all.

- first of all, you shouldn't make all levels accessible from the start. You'll have to make layer 2 events and such, but that would be a bit too much to explain here, but I can recommend you this very nice (a bit old, though) OW editing tutorial

To prevent moving up at the pipe is to place it 1 tile lower. Not sure why does that happen, though, but I tried it and it worked.

- You should fix the small cutoffs in the red circles.

And by the way..

There is an invisible block, for some reason.

Good luck on the overworld!

I was about to post a removal log for this as well, but noticed Hadron had already claimed it... I might as well post my findings here though, since I feel there's a couple of (minor) issues Hadron didn't bring up. (No offense to you though.)

First of all, adding layer 2 events might cause some of them to show glitched tiles when they occur. (Maybe that's what you meant by "OW glitches" and deliberately refrained from using events, who knows?)
Anyway, to fix that, make sure to click on "File --> Title Screen --> Uninstall Title Moves Recording ASM" in the Overworld Editor after you've recorded your custom title screen moves.

The ground tiles aren't connected too well here. Not too big of a deal, since everything works fine, but if at all possible, see if you can find tiles that act the same but allow for a less "cut-off" look.

Wooden blocks, due to not being perfectly square, cut off a bit of the tiles surrounding them. It won't be a huge deal if you keep it like this, though.

The pipe exit is positioned one tile too low here.

It's not a good idea to have unedited (or barely edited) levels accessible. Better get rid of them until they're done.