IMO unsolicited contributions and contributor entitlement (not to be confused with user entitlement) is a major issue in open source. Don't start working on a huge feature unless you've gotten the green light to do so, or prepare to be rejected. In fact, prepare to be rejected even if you've gotten the green light. Doubly so if you can't even write the code without help. It doesn't matter if you're doing work for free; unwanted work just waste both parties' time, and it's emotionally draining on both sides when the work is eventually rejected. Also, if you're just a random guy without a track record of solid contributions to the project, your chances of implementing an important feature "right" is not good. Ramp up your involvement with small features first. Build trust.
Personally, I'm no stranger to huge unsolicited contributions throughout my open source career. PRs that made changes everywhere, made the wrong architectural decisions, included tons of irrelevant commits and sometimes even changed existing coding style just for the hell of it. What should I say to the contributors? Often times it would cost me less time to rewrite the feature than guide them to fix their contributions. Often times it's not even a feature I would include.
A reminder that open source maintainers don't owe you anything, not even time to review your PR. Video conferencing to help you write your unsolicited PR is a huge stretch by any standard.
(As for "promises", it is indeed on the roadmap, under "planned for future versions": https://github.com/ONLYOFFICE/DocumentServer/blob/19e3a5d/Ro... Projects should totally have their own pace and priorities rather than blindly "listen to the community", and I've seen my own feature requests implemented after being on the roadmap for five years in software I use, so again, nothing inappropriate here.)