Some feedback/thoughts:
1. I don't think the current approach to difficulty is fun, its just feels limiting without any interaction or ways to turn it into an advantage. Some other ideas would be to have non-tetris pieces that are more difficult to place (but might be very good if you find agood placement), or temporary board obstacles that you can work around until they are gone/removed. Or pieces that do something else when placed like become obstacles until adjacent pieces are cleared. There are many ways to find a more fun difficulty approach :)
2. As others have pointed out it's frustrating to lose when you have "powers" left
3. The difficulty progression vs leaderboards are not clear to me. Am I only competing against players in the same difficulty? Can I choose a lower difficulty? (I would rather get the difficulty from competition than the fixed blocks on board)
4. Getting "holes" in normal tetris feels a lot less punishing than in your game. Here it almost feels unrecoverable at times. I don't have suggestion, and maybe it's not even a problem. Just wanted to mention it.
5. The powers feel a bit "meh". They are useful sure, but seem pretty similar in a way.