Just throwing this out.. For folks who really like the Favored Enemy feature, Pathfinder's Advanced Players' Guide has a Ranger spell called "Instant Enemy". It allows a ranger to temporarily apply his/her favored enemy feature to Any single foe for one minute/level. It's a level 3 spell, and probably still not as good as the Favored Foe feature. Would there be any use in adapting this to 5e?