Currently, shadows and assassins are the only melee based force users that do not have access to any lightsaber forms. I propose two forms, both of which would take advantage of the class' lightly armored, highly mobile nature.
Off the top of my head -
An offensive form (Jar'Kai perhaps?)- found in the madness/balance talent tree, The user becomes a whirlwhind of destruction, constantly pressing the attack and attempting to force the opponent to constantly defend and prevent him from having time to push forward. Effects could be something along the lines of increased alacrity and periodic stun effect (indicating an overwhelmed opponent with no opportunity to counterattack)
A defensive form (Makashi?) - available in the darkness/kinetic combat tree. The form focuses on mobility, footwork, evasion and counterattacks (which fits nicely with the concept of a lightly armored tank). Suggested effect - on a successful defense check, all damage that would have been done would be reflected back onto the attacker ignoring a percentage of his armor (to simulate being caught off guard with a successful counterattack)