I'm a newbie to posting, but I've been playing Dominion for many years. Playing with my friends is the best, and themed games are great fun, but randomization is the only way to go for rated games. It levels the playing field and ensures we are all being judged on the same criteria.
Now, if there were standardized themes offered that carried their own rating, that would be fine, too. I've seen customized rating systems in other games. That way, all players would be playing for a rating within each of those themes and the selected kingdoms/cards for each would be made known (and could be prepared for) in advance, perhaps in a separate tab. This kind of set up might satisfy those who want to play "more challenging" levels while still being rated, but only with peers within the same theme.
But let's not compare apples to oranges. As things stand right now, randomization is the only fair way to play rated games.