They would not even need to open source the servers. Just making the server available for users to run (even under a proprietary license) would be enough.
For any RPG (especially one with multiple characters):
Highly flexible keyboard controls to manage inventory.
I want text-editor levels of search, move, drop, swap, open, and close. Give me regexes, custom filters, and macros. Give me unlimited tags for items, and simple interfaces to manage them (eg: sell all that have a tag, move all items tagged with a characters’s name to their equipment slots).
It doesn’t need emacs keybindings, but that would be a big plus.