This list will be a short one, mostly because I honestly, don't want or think that Thundercasts should become the Next MOTUC line (in expansiveness levels). This is mostly based on the general consensus that most people only want the cats, main Mutants (Sorry Ratar-O) and Mumm-Ra. Seems that there isn't that much love for the Lunataks, the Warrior Maidens, or the Berzerkers.
We only need 2 and One of them is Cat's Lair. This is the "Castle Grayskull" of Thundercats. Not to mention prominent during the cartoon. The other Playset is not so much a Playset but a slightly oversized diorama...
(Because the Latin American dub is scarier than the Standard US voice)
Also, it's the other Playset from the Vintage Line... But I want ALL 4 ANCIENT SPIRITS OF EVIL!! There's no real need for an exterior since that would be a Pyramid and 4 Obelisks (The Obelisks you can get from one of those hippie shops that sell incenses and crap. The Pyramid is easier to make out of wood, foam or whatever.
Vehicles... That are NOT The Thundertank, which is the MANDATORY VEHICLE TO HAVE ON A THUNDERCATS Line:
Sky Cutter: This is the "War Sled" of Thundercats, you'd need at least 2. One for Monkian and the other for Jackalman... OK a Third One for Vultureman seeing that Vultureman's Flying Machine is not on this list.
Nosediver... Slythe needs his mode of Transportation, Yess!?
Final Vehicle? Thunder Strike... Lynx-O and Pumyra need a way to move when the Real Thundercats are driving the Thundertank... Bengali can rot in Hell for all I care.
No, I'm not counting the Kittens' Spaceboards as Vehicles since THEY are a MANDATORY accessory for the Thunderkittens. (but then again, we've seen how other toy companies MAKE GHOST RIDER walk... He's GHOST RIDER, not GHOST WALKER!)
Chances are that Most of this list will be not made, but IF they were to make playsets and vehicles, these are the most likely to happen.