What I would do.
1. Make SR like HA, start players off at level 3, passive xp, higher passive gold gain to remove 10 minutes of grinding, balance stacking champs and timed/stacking items accordingly and support items have finished quest.
2. Remove Runes and replace them with the old system but updated and more streamlined (no more keystones just stats).
3. Either 3 summoners or just make flash required for all champs and more summoner spells (snowball, remove stopwatch and make stasis a summoner, ranged knockback for disengage/counter gank).
Then just balance champions that are winning too much by adding animation clunk/delay > weakening stats in their kits so lower elo players can still play them.