This wish is really small, so I’ll add it to my list here: Beginner coding tasks in Krita: easy and short feature requests, ready to implement - this is a link that I always give people who want to start contributing to Krita, so there is a high chance it will be picked 
(especially since recently one contributor just picks one item after the other and I was just the other day joking I will have to make a whole new list soon 
)
Paying for a wishlist item to be done is something that Krita does, but quite rarely, and the costs are always way more than the person asking (if it’s a private person, not a company or something) expected and sometimes that they can afford. But in case like this one, it’s probably not even worth it since it’s a small task while it would require a contract and other formalities.
A whole system that would make this easier sounds like a good idea at first but I don’t think it would work out well. It wouldn’t be a donation, so Krita would have to pay taxes from it. It could skew the priorities in the direction of people wanting to pay and being able to afford that (and having credit cards that are accepted, etc.) - that harms inclusivity and it also harms the long-term plans of Krita. Krita once upon a time organized Kickstarters with goals selected by donors, and it was good, but then the whole work went to features and Krita was getting more and more unstable (that’s called “technical debt” - in this case, when you work on new things instead of making sure the things you’ve already done are finished properly).