What is the worst-case outcome from having a swollen spleen?

Rupture. Splenomegaly can cause rupture of the spleen, either spontaneous or with minimal trauma.Splenomegalys.
Splenic trauma. The worst case scenario is having the spleen rupture in an environment where medical help is not close by(falling while hiking in the back country). Splenic rupture is a surgical emergency and one could bleed to death without it. It is important to follow doctors orders: no rough housing with a swollen spleen.