What happens after you beat the final league?
I want to keep gaining coins and support candy for when the next league is released, but I'm worried that I won't be able to do league battles after I beat the last league. Obviously that will make it much harder to gain coins and trainer levels, so I'd rather retire my fish early and replay the last league repeatedly if that's the case. Specifically, I'm wondering the following questions:
Are there any league battles after you beat the final league? If so, are they as rewarding as the final league?
Will finishing the last league allow me to do battles that reward support candy and diamonds?
What happens when you beat the last league battle when your magikarp isn't at max level? Normally this lets you start the next league, but if there is no next league, that doesn't make sense.
Solution 1:
First off, there are three "expert" leagues after the final story league. They reward support candy and diamonds, plus they have a higher coin and exp reward than the final league. So you should definitely finish the last story league (currently the heal league) as you normally would. (Beat it before your magikarp maxes its level, if you can. I was able to beat nine battles in the first expert league with the same magikarp!)
Once you beat the expert league 3, your current magikarp is retired. Once you fish up your next magikarp, you start the same league over, just as if you hadn't beaten it at all. At that point, you can't get any more support candy or diamonds from battles, but you can get coins, exp, and random event rewards. There's no downside to finishing the last league sooner rather than later.
As mentioned, your magikarp is retired once you beat the last battle in the last league, regardless of whether it's at max level or not. The scene is the same as if you'd maxed it out, and you get both the giant exp reward and motivation bonus as normal. The only drawback is that it doesn't count towards either "retired at max level" or "forced to retire" in the achievements. If you're still working towards the former achievement, then you may as well level it all the way up. Otherwise, it doesn't matter if you beat the last battle at max level or not.
In summary, beat any and all league battles as soon as you can for maximum rewards, even the final one. You have nothing to lose by doing so.