I decided to look back at the PVE awards from December on (anything starting after Dec 1), to see if there is any discernible pattern.
There were 4 boss events (5 if you double count Civil War). 3 Latest characters and 2 Classic Characters were rewards. The Classic characters were XFDP and Luke Cage.
Otherwise: 25 events to date (through the current DPvMPQ).
4 of those were New releases, leaving 21 “open” events.
12 of those PVE’s rewarded Latest 4’s, 9 PVE’s rewarded Classics. This is in keeping with the normal history which has weighted PVE rewards towards the Latest 12 for a year now. (“Latest” didn’t exist until then.)
The order of Classics rewarded were (listed by event start date):
12/4/17 Moon Knight
12/7/17 Nick Fury
12/21/17 Spider Gwen
1/8/18 XFDP
1/18/18 Kingpin
1/28/18 Invisible Woman
2/11/18 Kate Bishop
2/26/18 War Machine
3/11/18 Drax
I can see no pattern from my end, except that Kingpin, Kate Bishop, and Drax all are/were announced as 5* feeders at the time they were rewards in a PVE, all around the same time of the month. There could be some rotation of Classic character rewards based on how long ago they were given out, except that the Boss event on 12/14 gave XFDP covers about 3 weeks before he was a PVE reward.
My best theory, based on this short list, that they try to reward less covered characters based on overall number of covers on rosters, since a lot of recent Classic rewards are middle tier and ones that a lot of players do not prioritize. Also, of course the 5* feeder is a thing and they are making mostly Classic 4’s feeders recently.
It is worth repeating from Granne’s post that the PVP 4* rewards proceed in alphabetical order, only interrupted by New Releases, and has been that way for quite some time. They even will reward a 4* that falls alphabetically not too long after their new release (as long as they are in tokens, that is).