(Warning; this post contains logical thinking based on what is presented in the show. If you are a Prime fanboy who will ignore all facts in the show to have your Gods back, please do it peacefully and keep in mind, Prime and it's characters' story is over.) Guys we're all idiots. Bumblebee and Ratchet have both made mention of being split up from Team Prime and not having contact with them for the longest time. Since Optimus was training in the Realm of the Primes for the longest time and hasn't been back on Cybertron since his return to Earth, it is safe to assume, the only Team Prime member besides Bumblebee he has had contact with since Prime, is Ratchet. Since Ratchet never had contact with any of the Team Prime members either, Optimus' group of old friends would probably only contain those off world he might now and Ratchet. Thus, they probably won't appear. Now just for sakes of confirming what left we have to deal with from Prime. To do this I'll list the status of all Transformers from Prime, dead or alive. °Autobots Optimus Prime - Investigating Cybertron's Government Bumblebee - Bee Team Leader Ratchet - Capturing war criminals with Undertone. Has contact with Optimus Prime after Decepticon Island. Arcee - Unknown. Probably living her life. Won't return. Bulkhead - Unknown. Probably living his life or working. He will return so I'll get to this later. Wheeljack - Unknown. Probably living his life. Won't return. Smokescreen - Unknown. Probably living his life. Won't return. Ultra Magnus - Unknown. Probably living his life. Won't return. Alpha Trion - Dead. Presumably in the Realm of the Primes. Cliffjumper - Dead Tailgate - Dead °Decepticons Megatron - Self exile. Might return. Starscream - Captured by Bee Team. Won't return. Soundwave - Captured by Bee Team. Won't return. Laserbeak - Captured by Bee Team. Won't return. Knock Out - Unknown. Probably living his life. Won't return. Shockwave - Unknown. Probably living his life as opposing the Government or trying something evil would be illogical towards self preservation. Won't return. Airachnid - Presumably dead on the moon. Breakdown/Cylas - Dead. Dreadwing - Dead Skyquake - Dead in the Shadowzone. Makeshift - Dead Vehicons - Some were Prisoners on the Alchemor. Some probably scattered throughout Cybertron. Insecticons - Many presumably dead on the moon, others like Razorhorn scattered around Cybertron. Hardshell - Dead Bombshock - Presumably dead. Unicron - Trapped inside the Allspark Container Nemesis Prime - In human captivity. Predaking - Dead Darksteel - Dead Skylynx - Dead As for humans, we don't know how long its been. They could be dead or the Bee Team won't contact them in order to hide from humans. Now as for why Bulkhead might return. He might be working as a Construction worker and met up with Ratchet or might be working off world and met OP. He might go to Earth for other reasons. As for the others who've been confirmed not to appear, Ratchet nor Bee have had contact with them. They're most likely living their lives and unaware of the council's actions or unwilling to start trouble. Probably the former. Megatron might return from exile and learn about the corruption.