Catholics seem to be increasingly well-represented among politicians, and as the largest denomination, with a bipartisan and multiracial base, more Catholic presidents would be a safe-money bet.
Since the question is new denominations, I think it's likely to be one of the Pentecostal groups, like Assemblies of God or the Foursquare Church. They might be downscale of mainliners, but being so large in number, upwardly-mobile members tend to get into the professional class and get elected as Republicans.
I'd also count Trump as non-denominational (rather than Presbyterian or Reformed, from his pre-presidential years), although it is, of course, not a denomination.
Trump is technically non-denominational but I would very much consider him a Mainline Protestant.
I think the next "new" denomination for President is certainly going to be either Jewish, Mormon or one of the smaller Evangelical denominations. Though that raises a good question - would I be right in thinking that we've never had an actually Evangelical President? Or would someone like Carter, Clinton or some one else count?
