Well the way it has been done in the past - before the great action figure collapse in the UK at least - was that every wave would contain plentiful supply of the current Doctor, at least one figure of the current companion, and then the rest would be any toy-worthy monsters from the most recent series. Now don't get me wrong, under Moffat I don't envy toy manufacturers, the monsters haven't exactly been toy friendly, and with the random gaps/mini-hiatuses in production that keep popping up I dread to think how any toy line could feasibly stick to that model these days let alone the fact that pops don't really target kids... But there's a reason they followed that vague plan, I mean somewhere in a box I've got a figure of a faceless little old lady, and Doctor Constantine (The Empty Child) because of that business model!
So for instance Series 1 did okay, not great but well enough for S2 to be released. They sort of got the mix right, and then S2 - immediately they start looking back. Ok, it was obviously an attempt to please the fans, but well the casual fans wouldn't have a clue who most of the people in Wave 2 were, and honestly a lot of the long-term fans are over-saturated when it comes to River, Jack, Rose et al even now 10 years later.
Honestly I think S2 should have contained Clara, Missy, and then the companions that double as cool monsters for the casual fans - Madame Vastra, Strax and possibly Jenny, and several more monsters (hell Daleks always sell - and the show itself has made a point of showing off just how many subtle variants of those there have been over the years; I'd love a cutesy version of the suicide-bomber Dalek from Destiny of the Daleks) as well as a variant of 12 - maybe Hoody 12. Eh - I'm not a toy exec, but I think where wave 2 failed was that it simultaneously managed to look too far back for casual people to want to pick the figures up, and not far back enough for the diehards.
S3 was an even stranger proposition with weirdly specific variants of already released figures, and Davros. Variants are cool, but honestly 11 could have had tweed jacket, green jacket/stetson, purple frock-coat before they had to turn to Mr Clever. I think S3 was a bit of a missed opportunity, given the long air-time gap S3 would likely have been the time to go from current-Who to classic-Who, and release a wave of classic Doctors, or perhaps gone all out with a wave of the Doctor's most famous enemies. Ranging from the actually fairly scary modern redesigns to things that are just amusingly goofy - so Daleks, Cybermen, Zygons, Silurians... And in a nod to the up-coming series possibly Movellans
Much as it pains me to admit it, older companions never sell as well as current Doc, current companion, current monsters/villains. Perhaps the older companions could have had their own separate chase/speciality line for the nerds, but well - CO who infamously made a huge number of figures never got around to some of my favourite companions (Romana, Turlough, Nyssa, Grace)
Hah sorry, bit of an essay, and my thoughts on the matter are probably full of holes, and I am very aware that Pops are a different kettle of fish/style of collecting than toys so there's likely a very logical explanation as to why the waves were laid out the way they were - but there's a damned good reason that the "Classic" range of figures in the other Who toy-lines tended to be more specialist affairs, and Jack, Rose et al were all long enough ago that these days that's probably what they count as.