I really think having to pay 11
to protect a permanent AND only as long as Terrasque is in play is much worse than PA, and 2 attack is nothing when facing any kind of shield or healing.
Most control decks have shields or healing or ways of removing a creature without problem.
Against aggro decks Terrasque is useless.
Ability to change the protected permanent is barely useful, especially in mono-earth, and I can't imagine a duo/rainbow deck eager to gather 11
just to protect some permanent.
My suggestion:
make Terrasue protect creatures instead. Something like "
: As long as Terrasque remains in play, target creature is untargetable. Terrasque cannot be made untargetable by any means."
This way he would offer something new, rather than duplicating PA. He would also have a nice synergy with plate armor, basilisk blood (you won't be able to protect any more creatures for 6 turns, but Terrasque is harder to kill) and Auburn Nymph.
It wouldn't be overpowered though due to his high cost and the fact you can remove protection through removing Terrasque. Rewind and mutation would remove protection from all creatures, while lobo, freeze or basilisk blood would only make him unable to use his ability to protect any more creatures.
Problems I see with this solution though: probably needs a lot of work to implement, and creates a new status similar to immortality and being burrowed.
So, if that's too much, then probably an ability to burrow any creature would be nice... especially if he could burrow opposing creatures too.