Red velvet is not going to vary much. It's a buttermilk-flavored cake that is more about red color and damp crumb (because it's oil-based) than a real discernible flavor outside of that buttermilk and vinegar tang. As it is now, it contains a negligible amount of cocoa powder.
Everyone here knows I pretty much hate red velvet, so I think if you want a more chocolatey cake, just make a devil's food cake and forget RVC.
Or make this: