I think they should keep some of the big names (Glimmer, Angella, Kowl, Lookee Mermista, Entrapta mainly) for the main sub. (Madame Razz and Scorpia should also be kept in the main sub). And some of these characters should be revealed at SDCC to help sell the subs.
I wouldn't mind if secondary POP characters are released in a POP add on sub of say 4/6 characters. But honestly, I'd much prefer another Filmation sub of 6/8 characters in 2014 with about half of the figures based on characters from the POP cartoon (eg Huntara, Micah, Vultak, Granita, Sundars, Hunga...).
Some multipacks could also make sense: a Brightmoon 3 pack (Glimmer, Angella, Micah), a Twiggets 3 pack or a small friends 2 pack (Lookee/Kowl).
The more POP, the better!
I'm really hoping for some big reveals at SDCC...