Mobile App
While I don't expect that extending Safari in iOS will ever be possible, and from a security perspective am glad that it isn't, an iOS app allowing read-only access would be invaluable.
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Upgrade Storage
will probably achieve some of that (actually both read&write) just by presenting an online interface - that's pretty far advanced I believe.
Otherwise there is a read and write API for Zotero, so people could start writing apps with relative ease - I doubt the core devs have much time, though.
I should probably do this myself, but I don't know if I have the time or resources to do so.