I think the most interesting of these are the Urophion, Tzakandi, Mindwitness, Uchuulon and Brainstealer Dragon since they seem to show that there is a very wide range of useable forms. The Kezreth mutated war-machine mentioned in "The Illithid Empire and the New Illithid Arsenal" implies that further changes can be made post-transformation.
Some initial rough thoughts were:
- The only limit that seems to be there is size - we see no ogres, giants, etc.
- If lizardfolk, grimlocks, gnolls and the wide range of goblinoids can be ceromorphised, why not aquatics - merfolk and tritons seem obviously suitable hosts, perhaps even loacathah, sahaugin and kuo-toa
- One would imagine that suitably adapted mindflayer colonies would be perfectly happy in the depths of the oceans, with extra miles of sun-blocking water overhead
- If a roper or dragon can be ceromorphised into Urophions and Brainstealer Dragons, why not a dragon turtle or giant crab?
- What about other large mammals with big brains - whales, dolphins? Are they tasty as is? What about an awakened one?
But testing this a bit more; mindflayers can move in 3D through psionic or magical power anyway so as a start point would prefer to dwell out of water to avoid being slowed down. The ideal state would be mountains and the surface in a place where the sun has been blocked or snuffed out. Where this has not happened and the baleful glare of the hateful day-star still dominate the sky, mindflayers may well choose the vast open ranges of the aquatic deeps. The sun is kept at bay by hundreds of feet of water, and they have greater freedom than the cave systems of the underdark.
Assuming our seas are populated with aquatics then there will be an abundance of tasty brains too. Our first difference is that it will be much more easy to tell who a mind-flayers base species was compared to the surface; merfolk, tritons and sahuagin look very different from the waist down.
So with that in mind, d20 things one might encounter at a deep sea mindflayer colony