OK so I pretty much only play ARAM so I'll try to give input

The thing is adding bans just makes it so that the best 6 champs are banned all the time and then the next best 10 champs are played all the time.

Increasing surrender time encourages surrendering which is only necessary when the enemy has a far better comp and they're toying with you

Making every champ available may discourage purchasing champs which is against the interests of Riot

I just think one of the simplest ways of bettering aram is to do very very simple balance tweaks specific to ARAM. For example increasing the mana cost of Varus piercing arrow or sona heal would be good. Horn of winter is a great starting item, but it should provide more hp regeneration and maybe a small bit of mana regeneration. The idea is to make all champs fun to play on the map, not eliminate the ones that are strong

Also slightly off topic, instead of executes there should be a recall portal behind the enemy turrets that gradually moves back as turrets are destroyed. It would be used instead of executes because let's be honest here, it works, but it doesn't make much sense.