I think it would be really cool if there were some rare hero portraits or even coins from doing hard achievements. Just like getting golden portrait for the default skin.
Currently there are some coins you can unlock by obtaining at least 1 of every card in a set, starting from Barrens and onward I believe.
Well I mean the golden portrait of the default skin is the original achievement, 500 ranked/arena wins with that class.
Plus you get 1 for reaching level 100 each season and doing something else that I forget. So personally I’m happy with them keeping the majority of cosmetics for £££ even if my favourite one happens to be the only one I technically paid for (Lady Vash that I got with whatever set that was).