I don't think there's THAT many, having in mind everything we've talked about so far.
-Japan will likely 3-0, at worst 3-1.
-The new players and the ones that didn't get to play are more prone to have a spot here.
-With that we have: Sengoku is a singles player, Kaji likely doubles (and he wouldn't be able to carry a match of his own story wise anyway), Kimijima is in a similar situation, Marui plays doubles, Mouri can play wtv but he is quite irrelevant and might not touch a tennis ball again in NPoT (this last part applies to the "three other" HSers too).
-Complete the line-up based on your other predictions and who you think isn't important/strong enough to play a match in the future.