I would be wary of just getting lenses because they are amazing
You'll end up with a lot of glass that way.
The A28 is a pretty great lens, and the A50 f2 is good enough, though not as good as the wider versions.
You weem to be building a good lineup with the limiteds. I suggest you experiment with the focal lengths you have, find the ones you prefer using, then (if necessary) complete your lineup with lenses in that range. For instance, if you find yourself using the 15 and the 77 the most, and want a zoom to serve the same role (not as good but more convenient) then something like the Pentax or Sigma 17-70s would be the best choice. If you want weather resistance, a faster wide zoom, a strong tele, the advice will be different.