Dota has predefined heroes with 4 skills (well, most of them) each. And that's the idea why it's so popular and rather balanced, generally. Dota would be broken, if you could mix hero skills.
You are comparing things that aren't as similar as you are insinuating. In DOTA there are like 100 something heroes to create all different kinds of combinations. There is no need to allow people to create their own because there are so many options, and the creativity comes from combining 5 of those options together against 5 opposing options. So, to take a step back fractally, combining 5 heroes in DOTA to create the most OP combos is very similar to combining 4 skills to create the most OP combos. It is just that combining skills is not necessary because there are so many heroes, it would just create unnecessary confusion.
In this game you only get to combine 2 heroes together. That is not many combinations, not much freedom to be creative and feel smart. In my opinion, if they don't either make 20-30 or more default characters to choose from OR allow people to create their own, this mode will just feel like a tacked on thing that you will play 5 or 10 times and be done with. It will never be a legit part of the game.
I like the idea of being able to create your own characters (at a set level and all that so it is conceivably balanced). But it carries with it the problem that the game has now which is skills are not balanced to begin with. And they don't really have to be because PvE. It also carries with it the problem of the confusion created because you have to know the spells that your opponent has to make the right decisions, so there would need to be a way to see all 24 or however many of them there are which is a lot of information to process.
