I think you should be able for each mod to get a special item from mobs that are for decorative use so you could make a collection room showing off all of the mobs that you have found. You could either have an item that you collect from a nob not just from killing it or you could have a way of collecting a head from each mob and you could then display them in some sort of way.
You already can make a collection room of mobs, just use living mobs.
That doesn't work as well when the mobs are trying to kill you. I think a mob collection system is a smarter way to handle it - you don't lose the old method should you prefer it, but you can also opt for a safer method that is still testing the same thing (finding and fighting the mob). Full Support!