I am really shocked no one has brought up Felix Cassander. Once he was on Fabius's slab the entire mini story captivated me. I was hoping when he got unloaded onto the Iron Hands ship he would have attacked Perterabo and got squished. Something heroic with him buying the loyalist enough time to do "Fill in the blank".
I tried googling him while I was reading the whole book because I couldn't wait to see what happened to him, and the rest of the book (while still good) was taking too long. Only book I have ever done that with.
While not heroic his end did generate some emotion with me. Poor bastard.
I suspected after "Betrayer" when Angron ascended they would do all 4 'specialist' Chaos Primarchs. "Angel Exterminatus" pretty much confirms that. The whole thing seems like a really big set up for the seige of Terra. We all know the only thing holding Chaos together is Horus, and without him... well things happened the way they happened. Having the envoys of the 4 Chaos gods there would strain that alliance all the more. I doubt any of the others will asecned until after the Heresy. We know Pert and Lorgar come after. Horus is dead. Alpharius, who knows (or cares). Curze (pretty much) kills himself.
And from a practical point, do you really think they are going to alienate the Nurgle and Tzientche lovers by not making them Daemon Princes during the heresy? I bet the authors are all fighting over who gets to tell Magnus and Mort's story. Please, anyone but Abnett!