Making it voluntary is fine. You'd always have a few people willing to pitch in. The rub comes when people start expecting things in return, or when staff wants to make it mandatory.
MU's exist in great numbers. There is always a free version of whatever you are offering. Basic capitalism says that this will go poorly. If you implement a pay-to-win strategy and give people extra perks, you're going to go against some pretty longstanding views about how players and their characters should be positioned.
It's generally not a bad idea to go 'here is a Patreon if you want to kick in a few bucks, and it is not required', making sure to show gratitude without showing favoritism.
But I don't think you're ever gonna get a play-to-play game off the ground. if people are gonna pay for a game they're likely to do it on an MMO or something with better graphics and more immediate returns.
MU's just aren't the model for profit.