I don't think attribute customisation would add much depth to the game and neither do I think that much depth is missing. you'd end up with a situation much like League of Legends where the mastery & runes systems are just a redundant way of disadvantaging less experienced players - redundant because their inexperience puts them already fundamentally at a disadvantage.
There are comparatively few indie games that achieve a massive breakaway success on a budget like GOIO.
The fact that GOIO is not one of these unbelievably successful indie projects does not mean the devs are doing something wrong. There are fundamentals of GOIO's design which I think leave it poorly equipped to maintain mass appeal: high learning curve, slow pacing; major focus on teamwork with little individual gratification; roles that are sometimes boring or feel unimportant to the outcome; long lobbies necessitated by the need to coordinate loadouts.
Most importantly I think, the game is really only fun when you're playing with fun people, and rapidly becomes unfun if even 1 or 2 members of your team are unfriendly or incompetent. Contrast to other team games like Dota which manage to be enjoyable despite a permanent assumption that your team will be assholes. that alone I think means you have to try quite hard sometimes to create the conditions in which GOIO can be enjoyable which sort of explains why it's quite common for people to lose interest quite quickly.
I think you have found yourself put off the game by one or more of these issues, probably most importantly never really making any friends to play with, and are now mistakenly pointing fingers at this supposed "missing" feature.
I guess maybe harder progression might motivate players to stick at the game longer and increase the chance that they discover the good side of it? but that could be achieved just as easily with a better designed soft-progression without compromising balance.
Even if a detail of GOIO's design has somehow handicapped its potential, there are a plethora of issues such as the eternally disappointing UI, never having a good matchmaking system, sloppy and inconsistent cosmetics, and lack of features to really mitigate the learning curve, that have played a more important role than the lack of hard progression. But again, I think those problems, while irritating, probably never wasted much of GOIO's potential because unfortunately its potential playerbase is inherently quite low because it's not a style of game that a great many people seem to want to play.
