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yup
I'll maybe try to give suggestions on how some of the modes could have been made more difficult without the inclusion of gravity, though these are just my guesses and i can't really test them or anything.

Long 1 was fairly balanced without gravity, technically possible to play for a while but it was a challenge. Having no hold helped with this.
Mini was death by gravity, without gravity it wouldn't be all too hard as you can just put mini blocks in one section of the field and big blocks in the other. (still a bit of balancing to do when you do this but it isn't that bad overall.) One way to make this mode harder without gravity could maybe be to change the playfield size, so that it was 10 mini blocks wide rather than 10 big blocks wide, and thus you have a lot less room to separate the two types.
Vertical was death by gravity, without gravity it would probably be playable forever as it was fairly possible to perfect clear (as you could make full vertical sections such as a 4-wide stack that all cleared at once.) A harder version of this mode might maybe change the randomizer to be harder such that it was trickier to rely on patterns that appeared in the bag? (such as playing-forever type stacks)
Twisted was playable forever even with gravity because it was essentially just normal Tetris in a weird field. There were some fun things you could do with this field like pattern building, but for normal play it didn't really affect much. Perhaps this could be made more difficult by making the line clears work differently to fit the field? (for example, you might have to clear a zig-zag line all the way across the bottom of both sections, rather than simply a horizontal line?
repeat tetrible was marathonable because most pieces you can just use over and over to build a giant 2 wide, and then throw the pieces you don't want down that. When you were playing freestyle and didn't have a good well set up, this mode was quite interesting, but once you were able to get a well you could maintain it forever til you messed up. The only idea I could really think of to make this one more difficult is to have garbage blocks occasionally appear to stop the player from building cleanly forever (perhaps similar to the garbage blocks that occupy the field at the beginning meant to prevent the player from starting out this way?) but i don't know exactly how this would be implemented.
letter tetrible was interesting and felt very similar to long tetrible, but you would always die once the gravity was unmanageable. the pieces were a challenge to stack with but were somewhat possible by combining the I and J into patterns. This mode might have worked on it's own without gravity, perhaps removing hold would be a way to make it more difficult.
lava tetrible was kinda cool since it incentivised stacking faster and stuff, this was very interesting for challenges such as an alternating secret grade pattern. however it wasn't very interesting for survival because you could always just spam hard drop to raise your stack. here, the speed of the lava was the challenge, and that feels like a fine challenge as long as it can actually kill you. perhaps this could work by making filled or even mostly filled rows sink slower into the lava somehow, such that spamming pieces would fall into the lava faster than you could place them, but placing pieces to create a more solid stack would make things sink slower? idk exactly how this would work.
a variation of this might even be to have it where lines don't clear, but you must fill all lines and if the lava consumes an unfilled line then you die or it starts going faster or something.
SUPER tetrible was for the most part normal tetris, except that the lock delay mechanics were very weird. The gravity applied when you moved the piece left and right wasn't really used much because the normal gravity quickly outpaced it. this was of course playable forever as long as you didn't screw up. One way this mode might be more interesting is by making there be no normal gravity, but instead the gravity that only moves when you move would start to go faster, until pieces could only have a very limited number of moves and rotates before they were already at ground level. for example, moving a piece from the middle to the side might already place it only a few blocks above the ground, forcing the player to be strategic and play very low so as to make sure moves stayed possible. This might echo something like killscreen play in NES Tetris where the gravity is so fast the pieces can't stack up very high at all (without hypertapping) except, here as time only moved when you moved it would be completely strategic.
tilted tetrible was playable forever because while the tilt was interesting, as long as you built a clean 9-0 stack it would not really mean too much. One way this might be a more interesting mode is by making the tilt it's own puzzle in that - the tilt would be the only way to move the pieces left and right. Pieces would be anchored to the right wall, with the only controls being rotate and hold, and they would be slid to their proper place. I don't know how long you could survive with this, but it might be an interesting puzzle not limited by gravity.
Double tetrible was a bit weird as gravity played a big part in this mode, but also a lot of difficulty came from the very buggy interactions between pieces. I'm not really sure how difficult this would be if there was no gravity. Managing the height of each piece and using Hold to try to reset this height was fairly interesting, though.

Remember all of these are just ideas, I don't know if any of these would actually improve the game modes, and there are probably other ways to make things difficult without gravity, just trying to give some ideas on how it might be possible to make interesting, non-infinite gamemodes that are based more on strategy than speed. Of course the speed modes can also be fine in their own way, but personally I do think that trying to find strategies that work well for each new version of the game each week is the most interesting aspect of all this.