What if Alaric I, the king of the Visigoths, survives in 410, successfully landing his army in Africa to control Italy's grain?
http://en.wikipedia.org/wiki/Alaric_I
This would mean the toppling of the Western Empire about 60 years earlier (in the sense that Odoacer toppled the Western Empire), establishing the Visigoths in Italy as the dominant power. Honorius is probably then toppled with the potential for a Visigothic Kingdom of Italy, much like the later Ostrogothic Italy of OTL.
Obviously the results of this are massive, as the Visigoths won't then be in Spain, nor will the Ostrogoths likely establish themselves in Italy. In fact they would stay in the Balkans, perhaps creating a permanent presence there. The Vandals may then become the dominant force in Spain, while the Lombards are then possibly forced north, around Italy into Southern France. Or they can try to challenge the Visigoths, but that may not turn out well for them.
A change this early then would probably butterfly Justinian I away, making Italy a Gothic stronghold, shielded from Byzantium by the Ostrogoths.