here are eight, just choose from three cause i wasn't sure what you would want
1. invasions by barbarian tribes
2. economic troubles and over reliance on slave labor
3. the rise of the eastern empire
4. over expansion and military overspending
5. government corruption and political instability
6. the arrival of the Huns and the migration of the Barbarian tribes
7. Christianity and the loss of traditional values
8. weakening of the Roman legions