Give us no more than 1 new champ each year
The more new champs you add, the harder it gets to balance the game.
I think you should focus more on balancing and reworking the champs we already have. Mordekaiser, an already existing champ, is more or less unplayable but instead of fixing him, new champs gets added.
Only release one champ a year, and bring a huge event with it. If a new champ from the Shadow Isles gets added for example, make a huge event about the place, and maybe add a rework with it too (Mordekaiser, hello). Doing this would also make it alot more exciting with a new champ. Right now I just feel "meh" when a new champ arrives. It really isn't that big anymore.