If you have a 28 and a 50 the other lens to have is a telephoto. I was in exactly the same position many years ago and bought an 80-200 zoom. It all depends on what you shoot.
if you do not like zooms get two primes: one 85 or 100 or 135 and another 200 or 300. And of course a good tripod to go with the long lenses. You will need it.