My wishes centre on internationalisation and localisation because I run a site which must give equal standing to two languages.
Allow admins to provide a FURL for every aspect of a URL. In 4.x the app names are fixed because they come from /applications/[app]/data/furl.json, meaning that non-English sites are condemned to have English words such as files and store in their URLs.
Allow admins to create and match equivalent pages depending on the language with their own slugs. For example, you have invisioncommunity.com/features/clubs/ on this site. If it were necessary for you to have a multilingual site, you would be able to create an associated page (very likely with the same content aside from text) with its own slug, such as invisioncommunity.com/trajtoj/kluboj/. When a link is created in the Menu Manager or written within content, the site would know that on click it is to load the language-dependent associated page; is somebody manually types <a href="invisioncommunity.com/features/clubs/"> within a post, the site would redirect to an associated language-dependent associated page.
Please move away from the approach of concatenating different language strings as a short-cut. What works in English often doesn't in other languages, and I'm forced to present grammatical mistakes in some parts of my site because words which are plugged in interchangeably in English strings take different forms depending on their grammatical role. My word for a post will have different forms in the superficially similar: {!#[?:%s liked]} %s and %s reacted to %s:, for example.
A long shot but please consider providing occasional context of how a language string is used. Translators need to know, for example, whether Report is a noun or a verb. (Incidentially, because of the previous point, I have instructions in an email notification inviting moderators to "Click to see this To report" because if I change to our equivalent noun, then the instruction "Report" on the content containers is the wrong word.)
It might be possible already for people more knowledgable than me but it would often be useful to be able to change the user's language setting via something in the URL. We're a UK-based charity so have English as our default language but our activity is worldwide using another. It'd be great for me if the language could be set as the second one for a visitor following a link such as invisioncommunity.com/cy for Welsh, for example.