Well, one of the main reasons for the prequels is to tell the story of Vader and Palpatine becoming who they are.
It would make sense for Lucas to show us all exactly why nobody likes the idea of facing Vaders or The Emperors wrath.
I think we'll see both Vader and Sidious show everyone who's boss. Sidious will destroy Yoda.
Obviously Sidious fights Yoda for a reason - to take out his biggest threat to his rule. Only Sidious can do it. He obviously thinks Yoda is dead - why else would he think all is well and they are home safe? The only concern Sidious has is about Luke Skywalker - so it's obvious he shows no worries about Yoda - he thinks he killed him 20 + years ago. (I am of course going by logic - I know Lucas is prone to not doing the same)
This will also establish Palpatine as a guy you do not screw with. He took out the most powerful Jedi at the time supposedly.
I'm more concerned about Vader's presence. Palpatine is coverred - he's gonna look like the biggest ass kicker when he beats down Yoda.
What about Vader? It troubles me even further that Lucas seems adamant that mech-Vader is a crippled man, rather than an upgrade. If human-Vader can't beat Obi-Wan - then how does suited Vader make himself look legit? The best way would be to have Vader show his true rage. The best way would be for Vader to take out his vengence on whatever Jedi he can find, fully suited, red saber in hand. It would not only look amazing, but it would solidify mech Vader as the walking demon we have all grown to respect.