Here's an alternate timeline I found on my message-board this morning and decided to post here for you:
Interauctoritas et Renovatio: A Roman Timeline
The point of divergence from our history is that Nero Claudius Drusus, stepson of Caesar Augustus, is only injured in a fall from his horse while on military campaign rather than dying. His presence as a viable heir means that Tiberius can't get away with, among other things, abandoning his responsibilities in Rome to go live on an island. As punishment, Augustus formally exiles Tiberius and later only allows him to return on the condition he stay out of politics, making Drusus his heir.
Drusus (supposedly) had republican sympathies, so upon Augustus's death, he decides to reorganize Roman government with a new constitution rather than simply pretending the old order was still intact all while maintaining de facto monarchical power himself. The new system retains an emperor who serves for life, but he is elected by the Senate from among the highest-ranking Senators, and many of his powers have been returned to the Senate. Furthermore, the Senate's control over the army has increased in order to prevent future military strongmen. The popular assemblies and other aspects of the Roman government are revamped as well.
Meanwhile, the longer life of Drusus has led to the incorporation of Germania to the Elbe River into the empire, which will have major ramifications for the future "barbarian" threat to Rome. There's also a reference to Drusus being remembered as a saint in "Roman Christianity" despite being a pagan, which means the divergence will have had a major effect on religion as well.
Overall, it's an interesting read, especially for those of you who are into the classical era. I'd check it out.