I'd like to monkey around with the speed of light in a vacuum, C. Turn it up a little, say, so that the energy derived from the sun's fusion was a little more, and we could see farther into past universe.
Or even better, make it many orders of magnitude higher, so that near-instantaneous communication between stars is possible; this would assume a decoupling of it from the mass/energy duality; maybe make it dependent on the speed of gravity, instead?
Or maybe a universe that was simply higher-energied, with the attendant ability to shape it. If C were on the order of 10^100, there'd be enough energy in the universe to make wormholes, exotic matter, and all sorts of other things possible.