Ghost Rider may have been out of sequence since that was when he was made a feeder for Gho5t Rider possibly.
The feeder came for Halloween, this is for Spider-Ham's release in December. It was not out of sequence.
General consensus seems to be Prowler as far as new characters.
Newer Essential for the N4mor release is Domino!A lot of people were complaining about Emma and Dazzler being regular essentials on the last trainings because they were rather new. Now everybody had enough time to collect Nebula - who would be next due to release order and for whom you could actually earn FOUR extra covers from Sinister 600 alliance progression - and they skip her!Personally I am glad because I don't have to spend iso-8 for Nebula. But again you learn that you can't predict nothing in this game.
It's because of the new fourstar essential pattern. They do the Three most recent fourstar releases (currently Domino, Bishop and Prowler), then two other fourstars, then repeat. One of the SHIELD training Essentials is the fourstar that's essential in the release PvE. Since Three out of five PvEs have a recently released character in that slot they are also overrepresented as SHIELD training Essentials.
But once Namor is released Domino should be featured a lot less as a PvE essential and thus will not be required in SHIELD training again for a long time.
It's kind of weird that Ronin is required for a second time, only about 3 months after being essential for Black Cat. I have no idea if there is a pattern at this point that we can look for.