Hello! Thank you for the questions. Here are the answers:
1) For the near future, I don't have plans to continue development on Mario 2.5. However, the source code for the game can be obtained at https://github.com/Intradom/Mario_25 in case you (or anyone else) would like to add new content to the game.
2) No story is/was planned for this game.
3) My original idea was to test out a dimensionality shift in the original Mario game along with some other twists, so there were no bosses planned.
4) I was hoping to test out different ideas and polish my development skills with this game for personal development. In terms of the game's development, refer to 1).
5) There definitely could be enemies in the future! Perhaps enemy goombas who can also go through the tunnels. However, as stated in 1), I currently have no plans to work on this game in the immediate future, so enemies would not be added in unless someone else were to do it.
Sorry for the disappointing answers, but I'm trying to develop my game making skills and test out new ideas right now, so most of my work will be focused on released small, prototype games that likely won't get many updates. However, once I feel more comfortable, I will try to transition to a bigger game with more support! If you enjoy my work, feel free to follow my Itch account!
Thank you for playing Mario 2.5!