I have thought quite thoroughly over just what Mayuri is working on and I have points to make about it before I tell my theory in this matter
1. Mayuri has locked the door and took away all his surveillance in this case he is working on something or someone that he doesn't want anyone to know about
2. It appears from the camara akon had hidden that it is most likely humanoid
3. if it is indeed a person in there then it is most likely someone who was too injured to get away from Mayuri (nobody is stupid enough to want to become Mayuri's experiment)
4. again assuming that it is a person we can determine that he/she is around Mayuri's height (174cm)
5. Mayuri is working on the shoulder/upper torso of whoever (or whatever) this is
Now from this I can see someone who fits all these points that I have made.Izuru Kira. He was severly injured in the torso/arm region. He is 173cm which is about Mayuri's height. Mayuri wouldn't want anyone (save Nemu) know that he is experimenting on a lieutenant hence the removal of the surveillance. And finally since he was injured it has not been stated that he is dead in this case why haven't we seen already where Kira is?
Your free to state any of your own opinions on this theory or any others about what Mayuri is up to. The feedback is good