It'd be an idea to nominate a hero to gain experience on whilst playing a different character, this could be a fresh character you'd never played before or a character you want to access the master skin on.

Hear me out on this one, as I know it doesn't make sense in the scheme of things game wise, but it removes the need to gate characters as you can do that with playing a character you're already versed in or the team needs and you feel like you have more control as to the progression direction you're taking. Very few will aim to have 10 on all chars but with this suggestion it would be a way to gauge your progress generally speaking without having to expand the current goals arbitrarily just because "once you hit 10, thats it for that character".

I feel people who feel forced to level each char to 5 are not picking characters for their worth in battle. Some characters are hard to play and I'm not suggesting to stick to 1 character but the problem with playing all classes is that you're putting your team at a disadvantage if you're playing a hero you're not familiar with all for the benefit of 500 gold (as an example) or to feed the OCD monster inside you because all characters have to be at a certain level.

Personally I think that sticking to a particularly good hero (read: one I feel fits my playstyle and I am successful as a result) and gaining lots of levels on that character is very unrewarding, I want to unlock more characters and use them when the situation arises but I feel limited by how many heroes I can buy with gold in a given amount of time. I'd certainly like to think that the ability to level one hero very high and at each level above 10, feel like I'm getting a fairly substantial reward towards my next hero unlock but I don't guess that's what's planned as it will directly conflict with their funding model.