Let me start off by saying the Mangekyou sharingans are great.
I thought it might make it ever better to have a mangekyou that you somewhat get a choice in. I'm not asking to let people pick which mangekyou they attain but they eternal mangekyou should depend on who's mangekyou was stolen to get eternal.
Ex. If I had Shisui mangekyou and I stole Sasuke's to get EMS maybe i'd get one or two of sasuke's mangekyou techs when i use my mangekyou sharingan.
Ex 2. Madara mangekyou steals Shisui and not only does his eternal mangekyou loosk lieka combination of both madara and shisui he gets kotoamatsu kami with his eternal mangekyou sharingan etc