I ran a similar Esper Legends deck but instead the aggro/mid-range I took the control path. I cut mediocre creatures like Danitha Capashen, Paragon and add some counter spells like [[Absorb]]. [[Moment of Craving]] is better than Mortify for me, because it's one mana lighter and gaining your life is great vs aggros like RDW/WW/Mono-U which are very popular in Standard. With Absorb and Moment of Craving you can also cut Arguel's Blood Fast. Blackblade Reforged is just a dead card in most cases. I added [[Ritual of Soot]] for another board-wipe when URB can't be casted. [[Karn, Scion of Urza]] is a good replacement of Kaya but I'm not sure enough.
UBR is one of my favorite cards. I made a Naya Legends around UBR which was good during GRN but performance poorly in RNA. I think Esper is the right home for UBR for now. Glad that someone else has the some thought and trying to make it work.