I do not believe GNW Sai nor Tenten will be sign-in rewards as they're both the "premium" GNW ninjas (ie. event gains).
I'll put my bet on Choji for now.

Well, considering that GNW Sakura, Lee and Temari aren't even in our Eng simulator I highly doubt they'd be in the monthly sign-in this year. Perhaps next year we can look forward to them. So far the ones we have are in the simulator (so is GNW Choji)
However... fingers crossed tho, if possible then I'd like to see GNW Sakura ^ ^