quicksand: The water weapon already blasts pillars. Of course, Pulverizer didn't stop me from making a deflag Mount, but I just don't like the idea of having the earth mount destroy pillars.
Plate Armor: Give any creature +0/+6 every turn? First, I think that earth has enough def-buff, with plate armor and bask blood, but secondly, I can't really envision a good reason for why the terrasque would buff your other creatures. Sure, some of the other mounts don't really make logical sense either, but it just doesn't feel right to me. It just doesn't seem as inovative as my other mounts.
Maybe the terrasque could have:
Dig. Burrow target creature you control.
The problem there is that there is no way to unburrow them then, but I guess it's a trade. He'd basically be a 'quint on a stick' then, making all your creatures invicible, but half-power.
I'm really trying to find something that really jumps out at me. I feel that this mount has been the most challenging, and it will take a lot of work to get this one right.
And btw, the pictures will of course change before these guys make it into the game. I'm not an artist. The pictures I've used are just examples for proof of concept.