The choice of content and adventures is determined by agreement between Roll20 and Chaosium. Chaosium controls the IP, and you'd be more likely to see action by petitioning them to release more of their catalog. In other words, it's less like Roll20 asks, "What modules do you have that we can convert?", but more like Chaosium says, "We would like to sell these products on your site." Then there are some meetings, contracts, etc., before a module can be scheduled and started. In truth, it's probably somewhere in between, but Chaosium is the one who controls the content. The best way to convince Chaosium to sell more stuff on Roll20, is to buy more of their stuff on Roll20, or to convince others to do so. Beyond that, write to them and express your interest.