Problem: The Timeless skin has already been deployed to several WMF wikis, but has a lot of bugs that need to be fixed, and it needs to be improved if it is to be used as a base from which to rewrite or replace Vector, which has a number of problems outlined by Isarra at this Project Grant request (which was unfortunately not accepted). Because of these issues, readers have been turning to websites with better and more modern reader-facing styling such as Wikiwand. Bugs are not currently being fixed, presumably because Isarra is busy in real life and no other devs have volunteered to fix them.
Who would benefit: Readers (so basically everyone, I guess)
Proposed solution: Finish the design review, fix the bugs, replace the colour scheme, etc.
More comments: This is similar to this proposal, but regardless of whether Timeless or another skin is used as "reader-facing", Timeless would probably end up being used as a base anyway because apparently the other skins' code is a mess (see the grant request).