From what I've researched (I'm not much of a figure collector nor know much about even the ones for KH) they went all out on the Series One PAs then after that they stuck to three up until series five when they released four figures but it was two of Sora and two of Riku.
Technically, the 4 DDD figures are broken up into 2 different sets and aren't together since The Grid figures came out together while the other DDD figures are coming out way down the road. However, people don't really know whether to put them into one volume or not. I just hope SE isn't dumbing it down to 2 per set. As for now (and until they release more PAs down the road), they've been sticking to 3 PAs a set and 5 FAs (Formation Arts) a set as opposed to 4 and 6 respectively which they used to do for each series.
I can't find a figure of riku or sora from DDD without it being 80+ US dollars.... I can remember when the originals were only 40 dollars (shipping included).
All SE merchandise went up in price after SE realized they were highly collectible and keep in mind, these are no longer just Play Arts figure, they are Play Arts KAI figures which are supposed to be more detailed and I think SE uses that as a way to justify the price. The figures have changed though. I just inspected all the Sora Play Arts figures released and the Play Arts figures are different from the Kai. The Kai are definitely more detailed and are a bit more realistic in terms of varying heights when I pose them all side by side.
P.S. This is where I am getting my The Grid figures, $55 each which isn't as bad as $80 each.
Kingdom Hearts 3D Play Arts Kai Riku Action Figure Tron Version at TFAW.com
i get the odd feeling that they're going to run out of Sora figures to make though... They can only do 4 more right now... Tron sora from kh2, merman sora, anti form sora, and sora from kh3 (IF he has any new suits....).
I don't think Merman Sora will be done but I wish he would be a figure. Odd Sora versions like Merman or Lion Cub are being done through Formation Arts, I do not think they would make them through their Play Arts line but I personally would keel over for an Ariel Play Arts figure (they did Jack, give Ariel some love).
for all we know, they could do vanitas and Ven and then say "No more BBS figures!". I guess it depends on really who they feel like making at the time... I can understand why they would make sora from each game, but I think they're just milking his figures to death now.....
Going by how they've been choosing their figures (purely based on popularity) they would make Aqua and Ventus IF they could only make 2. Aqua is easily the most popular BbS character by far if not one of the most popular KH characters thus far it seems not to mention BbS is, in the end, her story so making a figure of anyone else besides her for BbS isn't right. As for Ventus, they already have a Roxas template so all it would be is a change of clothes and Ven is popular.
i wouldn't mind seeing something like a series that includes something like Terra, organization 13 roxas with the oblivion and oathkeeper, and then Kh2 Tron sora. And then the next series could be like merman sora, vanitas, and xion.
I would enjoy Timeless River Sora, I have to admit, but the newest Formation Arts line is literally all the different world forms of Sora from KH2 (normal on DI, Merman, Halloweentown, Timeless River, Space Paranoids) and then they just had Pride Rock in the last series.
I also wonder on a Xion figure. I'm not sure they would make her, but I'd like one if they were released. I just don't understand why Terra wouldn't be put with Aqua and Ventus and Roxas with Xion. They don't need to do KH2 Tron Sora and at this rate, I don't think they will. They've already moved on past KH2 by this point and the Formation Arts are finishing up KH2 I believe with this latest set.
As for the Roxas figure, he wouldn't be put with Oathkeeper or Oblivion because they already made those 2 in the Keyblade Play Arts set, which is possibly the single hardest piece of SE official KH merchandise to find nowadays for reasons I have no idea why:
The only keyblade they've ever mass produced was the Kingdom Key because that one is relatively easy to make, but Oblivion and Oathkeeper are quite detailed to the point where they needed to charge more for them in this set along with the other 3 keyblade, all of which are very detailed. They stick out when placed next to the other PAs keyblades too.