Let's construct a possible Blitzball game bit by bit:
The first part is easy: We already have swimming, so you just fill an instance with water and have people swim in the boundaries.
Then you need goalposts. Put them on opposite ends of the instance, done.
Next is the ball and shooting. It would be fairly boring if you could just click the goal and press "score", so I'd set it up as vector based shot with a line indicator (effectively a slim tether between you and the point where the ball will end up). Then you can shoot the ball wherever, maybe score or if you don't hit, you can swim towards it, pick it up again and try again.
Then it starts to get messy: Allies and enemies. The first bit is easy - If you hit an ally, it's a pass. If you hit an enemy, he intercepts. If you hit nothing, anyone can pick up the ball where it lies. You'd need collision to prevent people from standing inside you to auto-intercept. And then you'd need some way to take the ball off a player that doesn't shoot it, either because he's AFK, trying to hoard or a bugged NPC (or because you just stand next to him and want the ball).
The easiest way is a time limit at which the ball is dropped automatically, but that's also pretty meh. You can make it a close combat skill (hookshot?) that has a random chance of success, but RNG is meh. An active time event of sorts maybe? It'd have to be client based, or else low latency auto-wins. I'd consider this to be the biggest hurdle, truth be told. It shouldn't be too long or obnoxious, but it'd be nice to have some player interaction there. I'm not sure what would be a good implementation for that.
Goalies, I dunno if it even needs them. As long as people need to be within a certain distance of the goal to hit it, I don't see the need, you just need some cheap super range shot prevention. If it does need them, I'd give that role to an NPC no matter what, because I know from PUG soccer that nobody wants to play it. Maybe as simple as a RNG chance to score that increases exponentially the closer the player is to the goal (up to guaranteed success). Not a fan of RNG, tho'.
For fluff, one can add trick shots that cost resources and can be adjusted for each character (or NPC). Just need to keep those balanced, but it can be used to promote passing the ball (give it to someone that still has resources), which would be a particularly good thing in group PvP.
For PvE, one can simply let the player control whoever has the ball in such a mode and have the rest on auto (or have everyone on auto and just control the player character, depending on control scheme). In addition, one could allow the player to give orders to the NPC on auto. 1v1 PvP could have two players each taking control of one team in that way, with the team without the ball being able to tab through characters (or keep them on auto). 6v6 PvP could be a fully fledged match with each player taking control only of themselves.
And at that point, you can tie in with the manager game and allow people to recruit NPC anyway. As you play more games, more NPC might be willing to join and you can build your team for PvE and 1v1 as you like. It's purely cosmetic in this incarnation, but it'd also be possible to give different NPC (and the player character) different stats such as throw strength, speed, endurance for trick shots or whatever.
Issues might arise from players not being displayed where they actually are - For wide open passes/shots, that'd be no big deal, the receiver can just move a little bit and pick up the ball. In tight quarters, that could lead to the wrong person getting the ball or unanticipated intercepts.
Another issue I could see in group (not solo) PvP is that players without the ball get bored (happens in real soccer as well). It's a consequence of the game mode, I have no idea how one could alleviate that except putting more than one ball into the game.
Another issue would be clarity: You'd need a lot of extra UI work to make sure people know what's going on in the globe. Something like round circles around characters and the ball, color coded depending on team.
Play-length, I don't consider much of an issue. I'd just cap the time as in all other PvP modes.
A big, big issue I could see, though, is NPCs being stupid. This implementation is fairly simple in concept, but fairly deep in execution and NPCs are very likely to be easily outplayed, follow orders awkwardly or in general have bad behavior. You can see in squadron missions how much they can struggle with much easier tab target gameplay and only two dimensions to move in. I'd see that as the breaking point of the design, because I'd see PvE and 1v1 PvP to be the most popular (due to being the most engaging for the individual player) and those rely on NPC.