John Lithgow is a treasure, but I agree. Gambon was 15 years younger when he was cast as Dumbledore. Richard Harris was 70 in Philosopher's Stone. Casting Lithgow is just asking for possibly having to recast in the middle of the show's run.

edit: My wildcard choice would be to roll up to Daniel Day-Lewis' house with a few trucks of money.

 

His wife would hate you if he agreed to play Dumbledore for 10 years.

 

I just hope they dont try to make Snape too likeable, the movies shyed away too much from showing his spitefullness and cruelty, and instead tried too hard to get you to like him.
He can look however you want, but I want to see him bully children, show his former past as a death eater(including getting the potters killed and calling Lily a mudblood) and not pretend he secretly liked Harry.