*deep breath*
This is a MMO, the underlying mechanics are that of a turn-based MMORPG in "server ticks" of about 3 seconds. Because of latency in North America, you can have players anywhere from 6ms in California to 500ms in Australia and Russia. Only the underlying turned-based mechanics allow such players to play together and not have the person with the fastest connection wipe the floor with everyone else. This is why action MMORPG's just do not work in North America and Europe, they only work in Japan and Korea.
Now, for Blitzball, or any other sport game, requires a real-time mechanic that can't be cheesed by the person with the fastest connection. So you would literately have players pointing to places on the field and pick "move,dribble, pass (to player), shoot (goal)" etc, and you would need whatever number of people are required to play the game. In FFX there were also stat boost/debuffs. Realistically FFX's Blitzball was also just turn-based. It was not in any way like the other sports games.
So the closest you will get to anything would be slower than the PvP games, and not as a fun as a result unless it was "solo", a la the squadrons. So between actions, you'd be allowed to move, but as soon as someone has the ball, or tries to intercept it, there would have to be some QTE for the success/fail action.
Like I don't realistically see any way of pulling this off and it being fun, regardless of what you think the value of Blitzball is itself. Look at the Triple Triad, Lord of Verminion and the Chocobo Racing. They are completely abandoned except when the tournaments happen, and even then people just throw them to get the rewards.
The "call a play" coach mechanic is easier to pull off, and not requiring assembling enough players to play, it could be done as 1-on-1, so whoever has the ball, you get a QTE of "pass/shoot/dribble" and that pretty much makes it work like it did in FFX. But that's not really much fun either. Ideally you get to pick other players who signed up (eg the duty queue), they get their team bathing suit, the players pick a number, and then those actual players initially get dropped into random teams until those with the highest skill get moved to an actual tournament ladder. If the players withdraw from the duty finder, they just get replaced by someone else in the same bracket.
But this is going to be something that takes a long time, you're not going to play a match and then logoff. If your "team" never logs on again, then what? Start over? What if your new team sucks? What if you get d/c'd?
Like "Venture" systems just result in you having nothing to actually do with the underlying game, and it's the easiest thing the developers can put in (like the squadron missions) and just RNG the result. But that's an entirely RNG based gameplay, it's not skill.
Hence, my point of I don't see any kind of "realtime" sport being viable here. It would have to be all single-rounds, not tournaments, and if you just PUG it, you stand no chance of actually getting anywhere since people could form "static" teams.
Anyway. I digress, I don't think Yoshi-P wants to spend development time on a full sports game within a MMORPG that people already barely play the PvP part of, and doesn't operate on game mechanics already in the game. So any implementation of it would likely end up more like the Squadron missions than anything else.