What should be the reasonable limit on Warshaper's Morphic Weapons class feature?
It says These morphic weapons need not be natural weapons that the creature already possesses. But, say, allowing a warshaper to spout multiple tentacles will be definitely too powerful.
The fluff for the entry does read, in part, "It's not until the solder you're fighting grows a tentacle and fangs that you know you have met a Warshaper."
And so, a tentacle sounds doable.
However, to specifically answer your question, I don't think the PrC allows you the ability to gain more attacks than you already have. A player of a Warshaper should not grow eight tentacles or extra arms and say "I'm going to make 10 attacks now, because I can".
I'd not allow the player to say "I'd like to make my hand into a poison stinger that deals Dex damage or venomous fangs, because that's a type of natural weapon."
If my player said "I want to turn my right arm into a Tentacle and my left arm into a Claw", I'd have that player look up the appropriate damage dealt by a tentacle of that character's size, a claw of the appropriate size and make the attack based upon the attacks the character can already make. If the player said "I want to make two tentacles erupt from my back to make attacks", I'd allow two tentacle attacks, but while those are attacking, the arms would not. If this player kept the two tentacles for the duration of the encounter, I'd have each round the player decide what two limbs are being used.
The Two Weapon Fighting feat or Multiattack feat would not suddenly be granted just because there are extra limbs. If the player wanted to actually take the Multiattack feat, I'd then allow an attack with a single extra limb as a DM call.
I'd not grant the Improved Grapple feat just because the arm is a tentacle.
If the player wanted to merge his hand into a ball of bone to deal bludgeoning damage or a long shard for slashing damage instead of a claws piercing damage, I'd allow it. I'd also be ok with a DM saying he or she would not.
