Before we make any changes driven by the BW2 enthusiasm, let's hold on. The metagame is just starting to evolve, lets at least let it settle. While I do like Latias, and I think it would fit well, many other new Pokemon could fit.
I am also with shrang about Arceus, Waterceus really didn't do much for me as I play tested it. I do hate using Grassceus in a sun team, so it might be the best to wait for BW2 metagame.