Thats assuming that these champs are introduced and never mentioned again.
The thing is though,that they usually do have some plans regarding these champs.
Rek'sai for example is known to have existed already during the time when renekton(and most likely nasus too) was human,so these 2 are linked,which can be seen in her lore section,they'll most likely plan to give us more information about this in the future.
Overall there are multiple stories that could involve rek'sai,I mean skarner being retconned into guarding something that lies underground in shurima pretty close to the release of a tunneling undergrouhnd monster that happens to live in surima too sounds like these 2 are going to clash at some point.Then there is Azir who is trying to unite shurima again,who is most likely seeing rek'sai as something that has to be eliminated.
The lore release is slow,but that doesn't mean that they don't plan stuff beforehand.
And while the league doesn't exist anymore,the city states and champs still exist.
So if any of those formerly imprisoned champs is going on a rampage,he'll probably end up like brand did(but if he's unlucky they won't bother with imprisonment).
Sure Cho'gath is powerful,but not unbeatable,he won't be able to bring down demacia or any other city state on his own.