A field twice as high as it is wide is scenes a faire. That much is obvious.
THe width of ten for tetris is actually very important.
A multiple of four makes the game foreverable under memoryless randomiser. Hence, 8 or 12 wide is out.
If you make the field width odd, then you are vulnerable to a run of squares ending the game.
So 9 or 11 is out.
So 10 wide and 20 tall really does appear to be the best.
You can tweak the randomizer to make other field widths playable, but the core game was developed with a memoryless one, and said randomizer influences the design.
AS long as the mechanic of clearing horizontal lines by filling them in, which is a core rule of tetris is used, the seven basic shapes and 90 degree rotation are a requirement to have a game that is playable, but not trivial.
The following things are, in my opinion, expression in a tetris style game.
1) rotation system. 90 degree rotation itself is scenes' a faire, but ARS and SRS are very different. The former is not part of the guideline, and is thus fair game i think.
Alterntive rotation systems, such as DRS have also been made.
2) piece colors. You can make them different easily enough, though making them bright and distinctive is scenes a faire.
3) lockdown system. We have move reset, step reset, entry reset, zero lock delay, etc. Move reset is what the guideline demands currently.
4) Number plus exact position of the piece previews. Moving them elsewhere OR changing the number is not a core rule.
5) scoring system. Scoring is not part of the core rules.
6) level system.
Note that TTC themselves has messed around with a lot of these.
The following are pretty much required for an implementation of the core tetris rules to be any good.
use of the seven tetrominos.
90 degree rotation
At least one piece preview
Lock delay (the days of gravity lock are long past, and nothing without lock delay can compete now)
Notably, the TGM series changes enough to be distinct from Guideline Tetris, and should not require a license from TTC. It shows the creativeity and redesign that Xio claims is not needed, yet still contains the seven basic shapes, the same field size, and all the core rules. HOWEVER, the TGM series is also an authentic tetris game, and thus under their protection somehow.
A game that clones neither the guideline, nor tgm should be in the clear, even if it contains only the seven minos.
The impression I got now was they cloned the guideline, and changes some of the colors. And if that's really all they did, then yeah I can see them losing.