My memory could be failing me, but doesn't the Shadowdancer get the ability to pretty much hide in plain sight (provided there are shadows) and travel from one to another at higher levels? if any of this is the case, getting the standard +1d6/2 levels sneak attack progression is definitely out of the question.
On the other hand, taking his existing class abilities - hiding, seeing through darkness, etc. and making them more powerful should be all right... Or perhaps replacing Summon Shadow with a familiar-type shadow creature that can't drain STR, but gives you similar benfits that a familiar does - eavesdropping, scouting, etc.
Hmm... What I would do in this situation, I think, is to allow you to make your own shadow animate and either strike out at your enemies (doing normal physical or energy damage, not drain) or stick close by you to protect you/intercept blows a certain number of times per your shadowdancer level for a limited amount of time... Make him something like, say, a Devoted Defender of 1/2 your character level, with the same stats as your character, maybe?