Kofi Kingston has waged an 11-year old war against convention and emerged victorious much to the delight of the WWE Universe, by becoming the current WWE Champion. Kingston, in all likelihood, will be waging another war against Kevin Owens very soon. There is no need for Roman Reigns to be inserted into this picture.
If Roman Reigns were to go up against Kofi Kingston right now, fans would abandon the product at once. It would not be unlike preparing a grand meal and then ruining it by adding just one ingredient that just spoils the taste altogether. WWE is riding high on WrestleMania momentum and will not want to spoil things.
Moreover, WWE does not usually like booking face vs. face programs, except for special circumstances like Rollins vs. Styles. WWE is riding high on KofiMania and the future is blazing with promise at the moment.