I'm not sure how you go about implementing your characters in game, but to allow people to make custom characters sounds doable with a LOT of hard work that you have to be willing to do. If you aren't, a custom feature simply isn't feasible.
Any real advertising costs money. It doesn't have to be a lot of money, but its never free. When it is free, it's not good advertising. I started advertising on Facebook for my business and created a website and shared content. Even then, it costs money for a domain and time to create the site.
There is no easy route to being a creator. If you want to successfully create you gotta put the time, work, and money into getting your dream working.