As an example, the team for Deadly Boss Mods could set a price of $1000 for the Ulduar expansion mods. Once they'd gotten that much in donations the mod would be released as previous versions were.
This should work for most people. Developers can be compensated for their effort. Blizzard can design content without worrying about "rich vs. poor player" issues. Players can get their mods at the same time as everyone else and not be bugged in game to send money.
There's still be some problems with it. Instead of mods having in-game advertisements we'd have raid leaders demanding people log off to go donate. This system would discourage regular bugfix updates in favor of big ransomable upgrades. But I think it's a workable option.
[Note: I picked DBM as an example because it's popular, given that they don't have a donation link on their site I doubt they'd actually be interested in this.]