What (besides cream cheese) could you frost red velvet cake with?

Vanilla buttercream is delicious on red velvet cake!

Swiss Meringue Buttercream. Not American.

you can use what ever frosting you like. Red Velvet doesn't have a specific flavor as it has a small amount of cocoa in it, so its not real chocolate flavor and mostly its vanilla cake with a little cocoa and a boatload of red food coloring.

ANY frosting you like will dress it up nicely.

Just a regular buttercream would be fine, Maybe your could sprinkle some freeze dried [crumbled] strawberries or raspberries over it?

Or add some berry color / flavor to the buttercream instead of vanilla?

Ask the bakery duh

I would select a Swiss or French Buttercream much more flavorful than the typical "American Buttercream" that is overly sweet and not much flavor. 

A visit to a "real" bakery and ask for a sample of a piece of cake with each type so you can get an idea what they are like. 

The French will be a bit richer than the Swiss.

they also both hold up well at room temperature or even in warmer temperatures. 

Frost it with 7 minute cooked frosting. Here is a recipe; https://www.kingarthurflour.com/recipes/seven-minu...

well technically red velvet is just a type of chocolate cake so what would you frost a chocolate cake with?

Buttercream will work,  Vanilla buttercream if you carefully brush off any crumbs on the cake beforehand (that's one of the issues with frosting a dark cake with a light frosting - crumbs)  Also, 7-minute icing" will work, but it almost has the consistency of a marshmallow!