The primary mode of time-based competition in the Tetris scene is undoubtedly the 40 lines mode, or Sprint as it's called in "official" terms. For some time, NullpoMino, a certain Tetris-clone, has also (sort of) been the de-facto game in which people compete.
Having been a part of the community for a while, and as a mod for NullpoMino here on speedrun.com, I've been noticing that there are other implementations of the game that people enjoy playing, specifically clones such as jstris, and Cultris II as well as Tetris Online Poland (private server of the now defunct Tetris Online Japan, and not a clone). To some extent, Tetris Friends and Puyo Puyo Tetris.
These games, too, have a 40 line mode, and people are achieving times somewhat equal to what they can achieve in NullpoMino. The current leaderboard of 40 lines mode on HardDrop does not discriminate between any of these versions, as long as the community deems it acceptable. Sure, some versions are imposing slowdowns compared to others by introducing animations and delays on line clears and piece placements, so there is no direct 1:1 comparison between top times in a game such as jstris and Puyo Puyo Tetris, but at that point it's up to the user to determine whether they want to compare themselves to the greater Tetris community, or just in their respective game.
What I'm trying to convey here is that the the NullpoMino leaderboard feels too limiting as other games are equally relevant in the competition. Would a general Tetris 40 line leaderboard be at all possible, seeing as it's not a specific "game" anymore?
That's likely what one would do, yes. The issue right now is that I don't know if I am allowed to request a mode present in several free standing games as its own "game". It does not make sense to modify any of the existing leaderboards as they are tied to their respective game.
I feel like what would be good here, is to make a "Combined 40 line Tetris" board that uses the Speedrun.com API to pull each separate Tetris game's 40 line board, and combine them in one monster list. That would avoid the hassle of trying to pull out one category from each separate game.
Your point here: "Sure, some versions are imposing slowdowns compared to others by introducing animations and delays on line clears and piece placements, so there is no direct 1:1 comparison between top times in a game such as jstris and Puyo Puyo Tetris"
Cross comparing data from multiple games with uneven timing condtions is somewhat confusing. The 40 lines times are often below 30 seconds, so slight timing differences are a large contributor. From my perspective, we'd probably rather not get in the business of making a board for a whole genre unless there's a very strong reason to do so.
I think what PresJPolk suggests is the best solution, and I've been told a leaderboard like that can be hosted on Harddrop (arguably the most prominent community for modern Tetris games) which is a pretty good starting point. I'm a little worried that it's going to flop for other reasons in that case, the abstraction away from speedrun.com potentially turning people away from the "effort" of registering/submitting, particularly if it's hosted on Harddrop which also requires the users to register to participate in the community. What I mean is that people are likely to register on Harddrop, but when presented with this theoretical leaderboard they might ask "why register twice to publish my record", or something. In that case, maybe it's better to just rework their attempt at a leaderboard without the use of speedrun.com's API.
You wouldn't have to make anyone register on a separate site. People would continue to submit to speedrun.com as usual. Your Harddrop page would pull the data from speedrun.com and aggregate it.
Your biggest problem is if some runners from game A think it's unfair to be compared with game B.