- Gains maximum hit points per HD.
- Gains a burrow speed of 10 ft.
- Gains a climb and swim speed of 15 ft. It gains a +8 racial bonus on climb and swim checks, and may take a 10 on these skill checks even when attacked or threatened. It retains its dexterity bonus to AC when climbing.
- Gains Blindsight, Tremorsense and Darkvision out to 300 ft.
- Immune to ability drain, blindness, deafness, death effects, paralysis, petrification, polymorph, slow and sleep. (In addition to its existing immunities.)
- The base damage of the tarrasque's natural weapons should be appropriate for a dragon of equal size, along with adding the crush and tail sweep attack options that dragons normally have.
- Mettle (Ex): The tarrasque can resist magical and unusual attacks with great willpower or fortitude. If it makes a successful Will or Fortitude save against an attack that normally would have a lesser effect on a successful save (such as any spell with a saving throw entry of Will half or Fortitude partial), it instead completely negates the effect.
- Rush (Ex): As a swift action, the tarrasque can move up to 2d6 times its speed. The tarrasque needs to wait 1d4 rounds after using its Rush ability before it can use it again.
- Reaving Strikes (Ex): The tarrasque's natural weapons ignore any damage reduction that can be overcome by any metallic material, such as silver, cold iron or adamantine.
- Rend (Ex): If the tarrasque hits with both claw attacks, it latches onto the opponent’s body and tears the flesh. This attack automatically deals an additional 8d6+25 points of damage.
- Exchange the six toughness feats for the following feats: Endurance, Improved Multiattack, Multiattack, Run, Steadfast Determination, Track.
I would make the following changes to the Tarrasque as it is listed here.