A solutiion could be that the world of evolving monsters is kept in check, there should always be the \"newbie monsters\" perhaps the farther away from whatever the designated newbie zone is the more likely it is that youll encounter bigger numbers of meaner monsters (of course there should be the possibillity that a new species would terrorize the newbie town until dealt with by stronger players)
So instead of just allowing these evolution models to run rampant and creating monsters which are ridiclously hard to slay, we put some control into it to provide for newbies.
Also we provide for the player whos been around years and is bored with the current monsters we have, he goes to his usual hunting area to kill a species he is familiar with, lo and behold a horde of something new, maybe faster,maybe stronger,maybe just new resistances come out and suprise him.
Then again we have to take into effect what this is going to do to the server resources. =/
Use them,
this is where I think the feasabillty for such an idea looses its steam, is a evolution model like this worth what its going to cost the server ?
Maybe so, then again maybe not.
Another possibillity is that we introduce monsters without having the evolution model in place,and reduce the numbers of the monster we want to phase out, its not exactly the same thing but it could very well have a similar effect
Personally I would like to see the evolution model in game but as always people thats just my opinion. =)