Possible ideas of Star Wars Prequel infinities:

TPM - What if Anakin hadn't destroyed the Trade Federation control ship? And what if Queen Amidala did not capture the Viceroy?

ATOC - What if Padme had been killed on the landing plat form? or what if she had gotten stung by those poisonous Kouhuns? Would she had died? And would Anakin want to avenge her death and get an early start down the path to the dark side?

ROTS - What if the Jedi had slain Palpatine before he could seduce Anakin? And what if General Grievous had defeated Obi-Wan on Utapau?

I would have also asked for ROTS "what if Anakin had defeated Obi-Wan on Mustafar?" but thanks to the bonus level of the Episode III game, we already have an idea.