Dunno whats wrong with the original critters, they are fine and enough
- You cannot change stuff like hair
- You cannot simply add new armors that look different
- You cannot add different bodytypes
If you now come with "you can edit the 2d images!" I will laugh. Everyone should know by now that it's nothing that can be done in minutes, hours, days, weeks or even months. Plus, the more - as example - different hair types you add, the more new frames you have to edit for a new armor. If you add new armor frames, you have to add even more new hair / head types, etc. It becomes more and more work and more and even more and a little bit more work on top of it.
Charactervariation is needed in a MMO. It's a problem that everyone is looking the same right now and not a feature. The only way to change this is to go with 3d models.
Beside this, the original critters have been 3d models as well. It's "just" needed to recreate the original character animations to make the new 3d models feel like the original ones. Most likely a different rendermode as well and some other tweaks, but it's not like it is something totally different, because... the characters in the original games have been 3d models too, like I wrote a lot times already.