I like these two sentences, they really turn out unpredictable, because they depend on the activity of users and on the value of posts by advanced users and everyone else too:
• When will the first user reach 10,000 merits?
• What year will the first user reach 10,000 merits?
As for the third sentence, (• When will user number 4 million register?) it really may not be accidental, especially taking into account such a fact, which was recently revealed here - Chinese BTT users opened many accounts simply for advertising purposes in order to promote some kind of their project. These actions may not be entirely random. So it doesn't quite fit.
I would say that it still adds a suspense but that is something that could be done very easily for free. Which means someone could wager a big amount for a date and then go out and create a ton of new accounts to make it reach at that moment. This will allow them to win by spending nothing, and it could be a problem for the bet, otherwise the idea itself is good if not for the fact that it could be manipulated so easily.
Basically merit is not like that, one can try to manipulate it as well but it will not be easy because getting merit is hard and it is not as easy as starting a new account. I think another one could be when will bitcointalk have its own token as well, I know that there is no talks about it right now but theymos could finally realize that creating a bitcointalk token to be used in the forum itself could be a lot of money for him and for anyone who uses the forum as well, so a token release date wager could be good too.