Cleon
Legend
Hmmm...
I'm almost thinking it would be easier to simply grant the imorph a +1 enhancement bonus to natural armor, a +1 racial bonus on saving throws, and a +1 racial bonus on attack rolls each round until it matches the morphed creature.
Thus, an imorph mimicking a troll would gain...
1st round: No natural armor (they are already the same), +1 attack, +1 Fort and Ref (Will is already superior to troll's)
2nd round: +1 attack, +1 Fort and Ref
3rd round: +1 attack, +1 Fort and Ref
4th round: +1 attack, +1 Fort (Reflex is now the same)
The trick is determining when it would take on the troll's attack modes and speed. We could just subtract the imorph's HD from the victim's to determine the number of round before full transformation occurs. Of course, that would make it gain the full troll abilities in one round, so my above example would be unnecessary.
I'm thinking imorphs should be able to take any shape, but only gain natural attacks and Ex abilities. So an imorph white dragon might gain claw, bite, wing buffets, and tail attacks, as well as icewalking, but wouldn't gain a breath weapon, spells, or SLAs.
We could but a HD cap of the creature it can mimic. Maybe double (or triple) the imorph's HD?
While I agree with treating the attacks & save bonuses as a racial bonus, I'd make the progression faster - although not AC, for reasons I've already given.
Maybe +3 attack, +2 armour and +1 saves per round.
A HD cap on the creature it can mimic makes sense but I'm not sure if it's worth bothering with since it may never take effect in play. They don't get extra HP, so it's highly unlikely they'd live long enough to imorph into a monster of over 10-15 HD, let alone a PC of such a high level (10-15th!).
Might as well throw the poor critter a bone and not give it an Imorphing HD cap, it'd have the advantage of making the power description less complicated.
As for copying natural attacks & Ex abilities, it depends how close you want to be to the source material. The original description says the monster does not adopt the mimicked creatures special abilities or attack forms, it just uses its own tentacle attacks although they look like weapons appropriate to the duplicated creature (the example is a pair of tentacle-attacks that appear to be a sword and a shield).
That still leaves the question of reach though, does an imorph tentacle pretending to be a longspear gain a 10' melee range?