January
Grøh
Nightmare
Xianghua
Kilik
February
Ivy
Zasalamel
March
Geralt
April
Siegfried
January revealed four characters, February revealed two characters, and March revealed one character. If we consider Siegfried's reveal to be half a character reveal since his other half (who he shares a body with but with a different moveset) was previously revealed as Nightmare (who we will consider a whole character reveal because we didn't know Siegfried was going to be in yet) then April revealed to us half a character. Going by this logic, Namco will reveal to us a quarter of a character for May.
If this theory holds true then the next character reveal will be none other than Elysium! She is a mimic character which can be considered half a character (because they don't have their own moveset) while at the same time copies only half the number of characters that normal mimics copy making her a quarter of a character. It turns out there really is a pattern to the character reveals!
All jokes aside, my guess is that Namco will reveal Maxi and Astaroth in May. At E3 in June, they will reveal Taki and Talim since they were the two most popular characters in the official character poll in addition to a new character and Yoshimitsu who made it to the top 10 in the poll and can also be used as a cross promotion with Tekken. By July, we should be seeing two of either Cervantes, Voldo, Tira, and Raphael.
My guess is that the pattern runs as 2, 4, 2, 1, 1, 2, 4, 2, 1, 1... which would total to the worst case of 20 characters by September. This would lead to an October release date with the rest of the roster (if we're getting more than 20) to be revealed during the same month before the game's release.