These days I assume that any PC who moves adjacent to a large creature and makes a melee attack against it has actually done something like wait for it to swing, then leap upon its forearm and clamber up to its collar, so he can stab it in the throat. I mean, if the monster fills a 4 x 4 area and can reach 2 or 3 more squares, clearly it's got limbs flailing around all over the place. It'd be almost impossible not to climb on top of the think as you fight it.
As for the tarrasque's mechanical design, if it's supposed to be a horrific monster that awakens every century and devastates the land, then let's do that. 'The land' is not just a physical thing, but a primal, spiritual entity. Whatever terrain the tarrasque claws its way across reels in agony. The air itself quavers, tearing birds from the sky and twisting winds into distorted gyres. Even the fabric of magic tears open, causing spells and prayers to manifest horrific backlashes against their casters.
Now boom, you've got inspiration. In addition to being a Godzilla brute with deadly claws and teeth, the tarrasque also creates earthquakes and whirlwinds, and its screams rend the very weave of reality.
Traits
* Ominous Gyre. Aura 100. A dark cloud of rocky debris whirls above the tarrasque at all times. Any that is at an elevation higher than the tarrasque's head (generally 30 ft. above the ground) takes X damage (some massive amount) and is blinded while within the cloud. Additionally, any creature that starts its turn in mid-air within 100 squares is slid 10 squares and must make a save to avoid crashing.
* Apocalyptic Upheaval. At the start of its turn, as a free action the tarrasque makes the following attack up to three times. It cannot target the same area more than once. Area burst 3 within 100. Effect: A stone spike bursts up out of the earth, raising the elevation of the affected area 30 ft. Any creature in the area takes X damage and is lifted upon the rocky shard.
When the tarrasque uses this ability, it can choose to have any other stone spikes it created on previous turns collapse. Creatures on those shards fall 30 ft. Creatures adjacent to those shards take X damage. The affected area becomes difficult terrain.
Standard Actions
* Claw. +x vs. AC. Hit: X damage and the target is grabbed. It must spend a minor action to sustain the grab.
* Vorpal Bite. Target: A creature grabbed by the tarrasque. +x vs. Fort. Hit: X damage. If the creature is bloodied after this damage, the target chooses foot, left hand, or right hand.
If foot, the character's foot is severed. The target takes ongoing X damage (save ends) and is slowed. Any foot slot items the character is wearing are swallowed. If a hand, that hand is severed. The target takes ongoing X damage (save ends) and cannot use that hand for anything. Any hand slot items or equipment held in that hand are swallowed.
* Thrash. Close burst of claws and tail.
* Ululating Agony. Recharge 4-6. The tarrasque roars, and the world screams in unparalleled suffering. Close burst 100. Hit: X psychic and thunder damage. The tarrasque may transfer a condition or effect affecting it to the creature. (Up to one condition or effect can be transferred per creature hit by this attack.)
Move Actions
* Hunger Can Wait. The tarrasque moves its speed and brings along any creature Large or smaller it is carrying.
Minor Actions
* Swat. +x vs. Fort. Target is pushed 5 squares.
Triggered Actions
* Cataclysm. Free Action. Trigger: The tarrasque becomes bloodied for the first time. Effect: Ululating Agony recharges and the tarrasque uses it immediately.
* The World Devours Its Children. Immediate Interrupt. Trigger: A ranged or area attack misses the tarrasque. Effect: Repeat the attack against another target within 20 squares of the tarrasque.
Something like that.
