The last two episodes have revealed that an unknown shadowy figure is reuniting cells of Codex around the globe. Russ Taylor doesn't think that it's Leland and it's more likely to be a new character. I think it will be a returning character, because introducing a new character would not be a huge twist.
Meanwhile, Jack Dalton is supposedly dead. However, we don't see his dead body and his death scenario is suspicious to say the least. Plus Gwendolen Hayes supposedly died and it was revealed that she became the head of Codex after being saved by Leland a Codex operative. Perhaps, Jack Dalton could have been saved in similar ways.
Having Jack Dalton return as a main antagonist with justifiable motives could be executed well. Plus, since hs knows how the Phoenix Foundation works Codex could use him in their plan.