In theory, the wiki Roadmap is supposed to indicate plans for the next major version. But this is often not kept up to date, and there are many PRs for enhancements that are never put up on the roadmap.
Maintenance releases have no overview of plans for what is included (other than the general policy to only fix bugs or enhance translations). But it isn’t too hard to count (manually) the number of commits since last release and bug PRs that might apply to the maintenance release.
I’ve never payed any attention to Mantis roadmap, and issues only get attention as some developer sees fit. For myself, I try to attend to bugs that I can actually solve, and that appear to be more or less likely to affect the user.
I’ve grown tired of trying to get major enhancements into place in the mainline Gramps, as several I have done are still pending after years of work. Which is kind of a disincentive.