WI: Rome fails to survive the Crisis of the Third Century?

Depends on what you mean by collapse, do you mean a total collapse into small kingdoms and wars in between the successors states or the empire splits into two parts, then things wouldn't be so bad.

If the Romans collapses into many warring states and warlords the first thing that crosses my mind would be the fact that Sassanids would be very happy for sure, they may extend and consolidate their domain in Armenia and may extend to cappadocia However, levant would be very difficult since there was lot of hostile elements of population against the sassanids and iranians in general. Eventually though the Sassanids would conquer the territories that would re create the realms of their Achaemeninds predecessor but that would take around 100 years and more.
My question is how would this effect Constantine? And later on the Hunnic Empire, and the pushing of the Germanic Tribes into these empires?