You've said Online Vs. is not in the plan, but I'm putting this out there with hopes that this somehow becomes real. GGPO Rollback Networking SDK (https://www.ggpo.net/) is free under The MIT License, here's rollback pseudo code (https://gist.github.com/rcmagic/f8d76bca32b5609e85ab156db38387e9), and check out Fightcade (https://www.fightcade.com/about) which lets you play Tetris Attack with P2P rollback netcode.
The demo is great. Good music, lovely visuals, and it even has gamepad support. I love playing on my arcade joystick. Not sure where else to leave feedback, but there's some game logic that doesn't quite work like in the original. Some other features I'd like to see: Button Mapping/Assignment menu, Tutorial Mode, Leaderboards/High Scores. Keep up the good work.