[1] Self-blockers.

    I don't like that a package X providing virtual Y can depend on
    !Y, but doesn't block itself.

[2] USE-flags in PROVIDE lines.

    Doesn't seem to make sense, or at least introduces a cycle.
    In use.defaults, some USE flags are enabled based on virtuals.
    If PROVIDE in turn depends on the state of USE flags, no
    proper resolution is possible.

    At the very least, the USE flags should be stored in the
    installed db in resolved state.

    Indeed, here's what we do: we use the USE entries of the
    installed db for this particular package to resolve the
    PROVIDE line. It doesn't seem to make sense to me that a
    package can dynamically stop to provide certain functionality
    if we change a USE flag.

    Ah, now I understand. It makes actually sense. If certain
    functionality is only installed if a USE flag is set, then
    a conditional PROVIDE is the right thing to do. Resolving 
    with the locally saved USE flags makes sense in this
    scenario.