I'd like to see some character customization, but through horizontal gear progression, where you have various options to push your character into a particular direction while not being strictly better or worse than other people at the same level

The problem I'd imagine would arise from your proposed idea, is it would either be an extremely long grind for just a small increase in stats, and if it wasn't, everyone would just level up everything... and be exactly the same as each other again, no different from what we have now