If you had to pick between balance and new champions...
I'll elaborate a bit better here:
Scenario 1: No new items/champions are added to the game. Riot focuses all of its resources and time into balancing everything to a pristine tune for the remainder of Leagues life; from balancing current champions and items, to balancing gold/distribution, minions, towers, jungle, exp- every detail of the game to "near" perfect levels (~95%). They begin to clearly define what beats what; for example Assassins -> Siege mages, but Bruiser/Fighter/Battlemage -> Assassins, etc.
Scenario 2: Riot focuses on constantly releasing new content. The game is ever changing and even more items, "new spells", etc. are being added into the game very frequently, to the point that you never know what the next patch will be like or how the meta will shift. Balancing becomes not even secondary, but tertiary and is a relatively low demand relative to their focus of pumping out content such as skins, champions, and things to keep the game "fresh".
I tried to write this without bias but I think its pretty clear which one I would pick; which would you?
we don't need more
just look at this guy