So this suggestion is pretty simple but it can be more and more complicated to allow more intricate automation processes, so I'll take time to explain it fully and leave it up to the devs how complicated they want to make it (if it gets added to the game).

So the basic idea behind this is a checkbox/"toggle button" on each character that will automatically level that character as soon as there is enough gold to do so.

The most simplistic way to do this would be to have it so that only one character can be auto-leveled at a time and it would auto-level until you turned auto-level off.

A more advanced way to do it would be have it able to be on as many characters as possible and it auto-levels them on a first-come-first-served basis. So if Champ A needed 50o to level and Champ B needed 55o to level, as soon as you got 50o Champ A would be leveled making Champ A take 56o to level. Then once you got 55o Champ B would level. And so on and so forth until you turned off auto-leveling for one or both of them (or added it to another champ).

With auto-levels, we could also add the ability to automatically buy skills. This would make it so that you could nearly fully automate most of the early-game.

With auto-levels and auto-skill purchases, wouldn't it be nice if we could also automatically buy champs as they become available? So auto-buy would be like auto-level.

Lastly, if you had a formation active, then when it bought the new champs it would be neat if it would automatically put them in the right place in the active formation instead of the first available slot (auto-formation).

Now that we have auto-level, auto-skills, auto-buy, and auto-formation the game is nearly able to be afked. The only way we could make this more complicated or feature-rich is if we could set limits on the auto-leveling... for example, say I know I only need to get Champ A to level 50 because that's when they get their party-buff skill. Well, if I put them on regular auto-level it would hit 50... but then as soon as I got enough gold it'd go 51... but I don't want it to go to 51 since that's a waste of gold. I would rather that gold go somewhere else. So ideally we should be able to set auto-limits. An auto-limit would be how high auto-leveling would level a champion until it automatically turned auto-leveling off. Because of the exponential nature of the game, this could either be a slider-bar (maybe one that auto-hides like the buff menus) or a field where you could input a number.

Lastly, since you can set all of these things, it'd be nice to have auto-leveling profiles (which could be linked to formations) that would save these settings. Then when you started a new run, you could just set an auto-leveling profile and let it do its thing until it needed human intervention (like if you got stuck on a boss, needed to use skills, got to a part you need to use clicking on, need to change formation, etc.)

So that's more or less the whole idea. Basically this would just make the early part of the game a lot more bearable because you could start it and have it go to level 100 without having to micro-manage your guys.