make the roman empire fall apart by year 300 or sooner
use colapsing social sistem, economic catastrofy, losing wars, civil wars beveen throne contenders, secesion of provinces, tetrarchy, infertility and desease, logistic problems, social upheval, uprisings, anithing you like realy, you dont ewen have to invent things, take things out of OTL, just make it fall, as in colapse, or at least split up into more than three states