My dream teams would be about seeing drivers from "my" time compared to each other in the same car, just because I'm curious.
Alonso and Kimi in the Mclaren MP4-20. To see how Alonso compared to Kimi in the same car, in a car that Kimi claims suited his style perfectly, and when Kimi was considered at his peak. I would still bet on Alonso, but most importantly, we would get an answer once and for all.
Or, Alonso and Schumi in the 2007 Ferrari, to see how awesome that car REALLY was, and an EPIC battle between the two.
Another fun one would be Mika and Schumi in the same car. Let's say a 97 Williams just to choose a car that no one of them drove. I'm pretty sure Schumi would win, but it would be very interesting to see how Mika compared speed wise in the same car.
And a final one: Alonso and Hamilton part II in the 2016 Mercedes.