I hope no, because I think that it will change all the serie (and I don't like changements)
If they tell anyone else their secret, who? Sissi? Well I hope no because it will creates lots of problems between Yumi and Ulrich (and I don't really like Sissi)
William? Arf, it would be the same problem and William is too pretentious :?
For me, it's good that they are 5 ;)

I have to that they might but the probably wont because they just can return to the past.