In all honesty, guys, what else could they have done? The fans wanted 2016 to be a Beast Wars set to honor the 20th anniversary, while Hasbro evidently wanted it to be a Combiner Wars set (as Fun Pub did state at this year's BotCon that they're now working closer with Hasbro in having Fun Pub's projects align more with Hasbro's plans). This set is even branded under the Combiner Wars label:
So with Hasbro having say over what they'd let Fun Pub do for this set, it more than likely that it was gonna be a Combiner Wars set regardless. And with there being such demand for another Beast Wars set to celebrate the anniversary, everyone would have complained if they had ignored the demand for more Beast Wars stuff.
So it would appear that they chose to go a route of fulfilling the desires of both Hasbro and the fans by doing a Beast Wars Combiner set. And since there just so happened to be a combiner in the Beast Wars toyline whose components did appear in the show but never got toys of their onscreen appearances, and whose fictional history had yet to be explored, who better to do than the Tripredacus Council?
I certainly wouldn't have wanted combiner versions of the show's main characters. And from all the backlash that the likes of CW Optimus, Cyclonus, Prowl, Sunstreaker, Mirage, Ironhide, Hound, Smokescreen, Wheeljack, and Trailbreaker have received, being called unnecessary additions to the line that "took away spots from the REAL combiners", I can imagine why the Club didn't go that route with this set.
So, it's perfectly understandable that they chose to go with the three council members and their two agents who appeared in the cartoon, as that was likely the safest route for them to take. I can understand the feeling of redundancy in their giving us another Pre-Beast Tarantulas so soon after the last one (I myself wish they had gone with another agent like Onyx Primal), but considering how the mass audiences familiar with Beast Wars most probably only know of the cartoon and are not as versed in the more obscure non-show Beast Wars fiction, the choice of show-character Tarantulas over some non-show nobody is, as I said, likely the safest route they could have taken. Ergo, a recognizable character is likely more appealing to most people than someone either completely unknown or someone known only to the TFWiki crowd.
On Episode #127 of the Twincast Podcast, someone made the suggestion of them having possibly instead made a combiner out of Beast Wars Neo Maximals Big Convoy, Break, Stampy, Longrack, etc., which was met with some positive feedback from the other podcasters. However, IMHO, that idea would not work as a Beast Wars 20th Anniversary set for two reasons.
1) Those characters are from a different Beast Wars show than the one that would be celebrated. That would be a Beast Wars Neo anniversary set, not a Beast Wars anniversary set.
2) Both of the Japanese Beast Wars cartoons (Second and Neo) take place in a completely different timeframe than that of the American Beast Wars cartoon. And I don't just mean how the American show is set in prehistoric times while the other two don't, no, I'm referring to the home time of the American Beast Wars show's cast. That "300 years after the Great War" setting that the American characters all came from before traveling back in time, that's not the same home time as the Japanese Beast Wars cartoons. Most of the Western fandom didn't know this at the time, but the Beast Wars Second and Neo cartoons established themselves as being set tens of thousands of years into the future, long after the "300 years into the future" setting of both Beast Wars and Beast Machines. So those Japanese characters are way off from the time period of Beast Wars.
Now, sure, some of the Japanese characters have cropped up in some of the 3H and Fun Pub Beast Era fiction (BWII Dirge in "Theft of the Golden Disk", for instance), but only in small roles and unattached to the events of the cartoons that they came from. And there's the Beast Wars: Uprising that just does whatever it wants with whichever characters, but that's a whole different setting from the American Beast Wars cartoon, which is what we're expecting this BotCon set to revolve around and celebrate.
To close, because what Hasbro wanted and what the fans wanted, it was inevitable that this set was gonna be a Combiner Wars/Beast Wars 20th Anniversary set. Yes, not everyone is going to be happy with the results (not everyone is ever always happy with the BotCon sets), but they tried with this one, and tried more than usual since we're getting a whopping five new headsculpts as opposed to the usual one, two, or three. And we're getting toys of characters based on cartoon appearances that never got toys before, and whose original toys were components of a combiner who hasn't received any kind of upgrade in 18-19 years. And aside from Tarantulas, all of these new toys are able to easily be integrated with all of the Pre-Beast Wars toys that Fun Pub has given us so far over the years. If you just plain don't like this set, that's fine, you don't have to like it. If you do like it, that's fine too. Just... Please understand what was trying to be accomplished here.