Well, they certainly need to be weaker than they are right now. But, hey, having upgrade options wouldn't be so bad if they weren't so uber to begin with.
The Greek Titan could start of on all fours, like a normal dog. One might be able to afford the special leg lifting attack - or even make it go bipedal for superior aiming.
The Isis Titan could have a high-heels upgrade, adding naughty spice to that devastatging stomp attack.
The Norse Titan could have the Belch of the Titans upgrade, which increases damage and has an effect on buildings like Pestilence.
The Atlantean... hmmm. Reflecting the nature of the race, perhaps a flatulence upgrade. The special attack would make units run and effect those in extreme proximity much like shockwave.