Season 4 Rumours that have drifted my way...

• Lena may be heading towards a more adversarial role, against Supergirl. The break between her and Kara will happen in episode 20, and it will be very emotional. In a major (and lengthy) scene Lena will learn Kara's true identity, and be so upset that she literally walks out on Kara without listening to explanations -- no doubt this will have some influence on the break up. Lena (a bit like Max in S1) starts to become more distrustful of Kryptonians. The rift between her and Kara is still left unresolved at season's end.
• Ep 14 revolves around Winn and the death of his father (Toyman) in prison , which causes Winn to question if criminal insanity runs in the family. Winn's identity as the son of the Toyman is revealed to the public, resulting in a hate campaign against him. Lena and Winn become friends.
• Mon-El and the gang switch to new costumes that look more like traditional super-heroic suits (episode 15.) Mon-El's red suit was apparently made for Chris Wood last season, but not used. The costumes somehow enhance the Legion's superpowers, and they'll be a regular sight in the final part of S3,
• The Legion will leave on a mission into the future in ep 17, and return in 19, with flashback used to explain what happened. The flashbacks will feature more Legion members than we've currently seen. Kara won't go with them and indeed stays in the present for the remained of the season.
• James is injured in episode 19, and out of action in 20, which allows Lena to get up to no good unchecked. Lena dates James and will learn his identity as The Guardian, and takes the revelation well.
• Alura (Kara's mom) is in part responsible for the world killers.
• Cat Grant may guest star before the season's end (ep23, or before?), and Superman too (eps 21 and 22 only -- but not ep 23.) Eps 21 -- 23 have yet to finish being written. Ep 23 may be largely a wind-down/loose-ends episode, with most of the finale action happening in eps 21 and 22.
• Chris Wood has a big fight scene in ep17.
• J'onn's dad gets seriously sick in ep15, and Alex spends some time comforting J'onn -- perhaps he dies(?)
• Imra's secret is revealed in ep 16. She breaks up with Mon-El, but that doesn't mean that Kara and Mon-El will become an item once more (two separate sources agree that Kara has no romantic storyline this season.)
• Odette (Reign) won't be in S4, as she already has signed to other work commitments on another series.
• One original plot idea was for Alex to eventually adopt Ruby, but that seems to have changed. The SG staffers really want Floriana (Maggie) back, and have been trying to negotiate something. (Was the original plan for Alex/Maggie/Ruby to form a family is S4???)
• The show's staff are hopeful for a fourth season renewal.

• When it is revealed that Sam and Reign are the same person, Ruby is taken to the DEO.
• Mon-El is 'staying' at the end of the season (which I can only assume means he will remain in the 21st Century.) The jury is still out on whether Kara and Mon-El will rekindle their romance.
• Still no deal with Floriana to return as Maggie. If she does, it can only be in eps 22 or 23.
• Erica Durance will return in flashback as Alura to explain the worldkillers creation.
• Lillian Luthor is in episode 21, and has scenes with Lena. The episode involves action on a boat (!?!), possibly concerning Alex pursuing Lena and Lillian. Elsewhere in ep21, Kal-El and Kara are teamed up for part of the episode to deal with the worldkillers.
• Lena doesn't actually hate Kara, at least not this season. (Presumably the fallout between the two is more about a sense of betrayal??)
• People who won't be returning in S3 pt2: Jeremiah Danvers and PSI.

I also now have a better sense of roughly where in the production pipeline the sources claim to be getting their info from.

• Imra's secret is that she and Brainy have travelled to the 21st Century to rescue Imra's sister (who is also a time traveller) from being killed by Purity.
• Sounds like the Alex boat(s) scene in ep21 is a big spectacular action sequence.
• Episode 17 features Pestilence (the third worldkiller) who is a pediatrician, so the DEO try to use her love of kids to counter her darker urges.
• Episode 18 has Lena finally reveal Sam's secret to someone. She recruits Lillian to cure Sam, but things go badly and Lillian is almost killed by Reign. Lena and Lillian bond, giving Lillian the chance to plant a few seeds of doubt in Lena's mind about Kryptonians.
• Tentative plans for ep21 (open to change): Reign kills Purity, after Purity goes berserk (imbalanced) resulting in the death of bystanders, including Imra's sister. Reign also (at some point) battles Imra and Imra gets pretty badly beaten, although she survives -- perhaps this happens in ep21(?)
• Tentative plans for ep22 (open to change): Purity's death causes Pestilence to also become unstable.
• It sounds like Lillian might clue Lena into the fact that Supergirl (like Reign) may also have a dual identity, and Lena starts to suspect her friend Kara. The penny finally drops when Lena and Kara next meet -- Lena keeps fishing for clues, and Kara finally confesses.

• It seems that the secret of Reign's dual identity may come out sooner rather than later. Episode 14 or 15, perhaps? (Source seems very vague... and it doesn't make sense for the DEO to know until closer to episode 18, unless Sam goes into hiding for a few episodes.)
• Ruby finds out that her mom is Reign in episode 18. It is in this episode that Alex takes Ruby to the DEO for protection. Reign tries to recover Ruby. (Speculation: perhaps this episode is meant to have a deeply ironic storyline: Lena/LexCorp are close to curing Sam, but the DEO's actions by taking Ruby unwittingly thwart them by triggering Reign?)
• Abbotsford Mansion will be a filming location (perhaps the Luthor family home -- seems a logical possibility?) Tweet

Unknown episode info

• At some point Lillian causes Lena to suspect that Kara may be Supergirl -- ep 18 maybe?
• At some point Lena learns James’ identity as Guardian -- ep 19 maybe?
• At some point Reign battles Imra, who is pretty badly beaten by Reign.
• At some point Alura will appear in flashback, to explain her part in creating the worldkillers. Kara’s dreams are also connected to her mom’s involvement.

Episodes

Ep 14: Revolves around the death of Toyman, Winn’s father. The secret of Winn’s connection to him is revealed to the public, resulting in a public backlash. Lena comforts Winn.

Ep 15: J’onn’s father gets very ill, resulting in some very emotional scenes were Alex helps J’onn cope. Mon-El gets his new costume in this episode -- the other Legion members also get costumes, which enhance their powers, but they may not be revealed in this episode (ep 16?)

Ep 16: Imra’s secret is revealed: she and Brainy have travelled to the 21st century to rescue Imra's sister (who is also a time traveller) from being killed by Purity. Imra and Mon-El break up (however there won’t be a rekindling of the romance between Kara and Mon-El this season.)

Ep 17: Introduces Pestilence (the third worldkiller) who is a paediatrician, so the DEO try to use her love of kids to tame her darker urges. The Legion journey into the future in this episode, returning in ep 19. Mon-El has a big fight scene in this episode.

Ep 18: Alex takes Ruby to the DEO after Ruby learns that Sam is Reign (it is suggested that the DEO already know by this episode.) Lena recruits Lillian/LexCorp to cure Sam, but things go badly (speculation: as a result of Ruby being taken to the DEO?) and Lillian is almost killed by Reign. Lena and Lillian bond, giving Lillian the chance to plant a few seeds of doubt in Lena's mind about Kryptonians.

Ep 19: The Legion return, and their future tales are told in flashback, featuring Legion members who we’ve not seen in the 21st century. Also in this episode James injures himself, and is out of action for ep 20.

Ep 20: Lena’s learns of Kara’s secret identity. Lena already suspects that Supergirl has an alter ego, like Reign, and (in a major scene) begins to drop clues to Kara. Kara finally confesses. Lena is so upset that she walks out on Kara without waiting for explanations or justifications. The rift between her and Kara is still left unresolved at season's end. Although Lena is upset, she does not become evil.

Ep 21: Superman and Cat Grant guest. The episode involves a sequence on a couple of boats, possibly concerning Alex pursuing Lena and Lillian as part of a big action sequence. Elsewhere, Kal-El and Kara team up for part of the episode to deal with the worldkillers. Tentative plans: Reign kills Purity after Purity becomes imbalanced, resulting in the death of innocent bystanders, Imra's sister being one of them.

Ep 22: Superman guests. Tentative plot: Purity's death causes Pestilence to also become unstable.

Ep 23: Tentative plot: wind down episode. The Legion leave, but Mon-El stays in the 21st century.

Next season

• Odette Annable has commitments that mean she won’t appear in S4.
• Mon-El stays behind, but Imra and the Legion leave.