Aly now has the synopsis posted on her blog: Red and the Task Force spring into action after learning who betrayed them. 

Interestingly, the title is TBD: Conclusion (Pt. 1). 

Normally when we get double episodes, the first one is titled [NAME] and the second [NAME: Conclusion]. Since the first one is already titled Conclusion, it seems to me that the final blacklister of this season will be someone who has been on the list before. Of course this leaves a number of options, but most likely it will be one of Red's colleagues or an old foe looking for revenge. Based on the active members relatively high on the list, I think the following would be possible (though most of them unlikely): 
- Marvin Gerard (heavily speculated, but he's low on the list compared to most finale blacklisters)
- Dembe Zuma (unlikely but that would be a twist. Also he's high on the list)
- Mr. Kaplan (strongly hinted but somehow unlikely still. Also we had a conclusion for her already)
- Elizabeth Keen (even more unlikely since we saw her die)
- Katarina Rostova (perhaps the real one will show up, though this seems unlikely given the past season hints. Also we had a conclusion already)
- Ivan Stepanov (unlikely for various reasons)
- Alexander Kirk (unlikely and we had a conclusion ep. already) 
- Robert Diaz (unlikely act of revenge, but a far-fetched possibility if he's linked to the Cabal)
- Godwin Page (unlikely given his irrelevance and lack of power, but Townsend's death may be a motive)

I hope this post is in line with the Forum's rules. I thought it would be fun to have some speculation, which is why I added some wild and unlikely options to the list.