I want to address 2 things here. first up, the dog, second up, classes.
Firstly, while I wouldn't necessarily be opposed to a canine addition, I think it would be impractical, largely due to one big limitation: Lack of opposable thumbs. What weapons would a dog have? I can already see many working ins, such as another means to deploy urine (perhaps a refill station for snipers), the stun on sandwich use, etc. However, claws and teeth don't make for much variation; it'd all be melee. How about unlockables? bigger claws? silver teeth? A beehive attached to the roof of their mouth (read: watch the simpsons)?
As for the issue of class, I think that 9 is likely enough. If you think of a full team of 16, there is already a particular sort of imbalance with the classes; on a strict level of number balance, there can't be even numbers. you'd need a team of 18 to have 2 of every class. That said, in practice, the classes are never balanced that way. on solid team, you'll have more engies, more demos, perhaps. On a typical team, you'll have all pyros and snipers, but that's another issue.
Yes, I believe it may turn out to be too crowded, but in practice, I'm more worried about role. we had, once upon a time, a thread about what another awesome class would be, and there were some cool ideas; however, it would mess up the fundamental '3 O, 3 D, 3 support' structure that is in place currently. What would they be, another support?
I believe wholeheartedly that they will not be releasing another class until they finish the current round of class packs (that is, giving the demo, engie, and soldier their packs). Even when that's done, I think they're more likely to release a SECOND set of class packs for everyone before they make another entirely new class.
My 2 cents