I think we must have a special-like overworld, as every level has a different theme, and preferably revealed in sequence so it's guaranted every level is played. We must decide the order based on Najeraldo's feedback on every level.
About the title screen, yes, we should use level 105. The intro level screen must be changed, but I think the message is ok (although it was supposed to be a joke many people approved it, and since this hack doesn't have plot it fits enough)
Finally, vanilla credits are enough. However, doing a credits sequence ala SMB3
would be nice (instead of every world, a sequence with a part of every level and every person who did a screen on it) Don't know if doing this could be difficult though.