Relation between versions: versions can bundle other project versions
It's about Roadmaps and having an overview about which modules get bundled with which product versions. There's currently no built-in way to link versions, and I think it would benefit Redmine.
One can already create custom version fields, and they would already be displayed by the Roadmap as links, but it could be more, especially where one needs a central database during software planning about what module is bundled with what product. Plus, if a module is delayed in version planning, then it could indicate that it delays a product release in product Roadmap. In the module Roadmap it could display in what products the module is bundled and whether it delays anything. 'Product' and 'Module' are plain projects in Redmine, but have different semantics and are used by different teams, planning and development, respectively.