Daredevil fans were given a gift starting in 2015 with the MCU's loosely-connected TV series on Netflix. It showed masterful quality particularly in seasons one and three, though even the 'black sheep' season two was generally well received in spite of its flaws and being outclassed by the giants that seasons one and three are. That's why November 2018 came as a bitter sucker-punch when internal, corporate bitterness and competition (Disney+) forced Daredevil to be canceled despite season three's critical acclaim.
Daredevil was by far the best of the bunch and a genuine critical success, making the cancelation and the couple of loose ends left even crueler. Fans saw a teased return of Bullseye at the end of season three, and Elektra's body was mysteriously missing after the events of The Defenders. Fans will largely favor Bullseye's ominous return - and understandably so - but here is why both deserve resolutions.
Bullseye: Biggest & Most Recent Cliffhanger
While perhaps partly due to it being the most recent, the season three cliffhanger teasing a looming return for Wilson Bethel's Ben Poindexter/Bullseye is genuinely, largely seen as the most significant cliffhanger of the series. Yes, Elektra's 'death' at the end of season two was a major set-up for The Defenders, but given we've moved past The Hand, even her ambiguous fate at the end of that series and her/her body's disappearance in the wreckage isn't as urgent as what Bullseye was preparing for.
The latter brought a new, more intimate/personal, dangerous, and horrifying threat to Matt Murdock/Daredevil, the main cast, and Hell's Kitchen generally, so seeing him prep a comeback demands a resolution in a hypothetical season four.
Elektra: Among The Best Dynamics/Relationships From Daredevil Comics
Similar to Batman's complicated - to say the least - romantic, on-off relationships with Selina Kyle/Catwoman and Talia al Ghul, Daredevil's relationship with Elektra is at least as complex and just as dangerous. She brilliantly mixes the mutual, tense attraction of the Batman/Catwoman romance dynamic with the unpredictability in lethality that Talia al Ghul (also a member of a ninja cult) brings to the Batman/Talia romance.
Daredevil and Elektra have a deeper past together and have been on-off, friend and foe several times, so bringing her back and exploring that deeper given where they left off would be immensely-satisfying to see.
Bullseye: Daredevil's Second Biggest Supervillain
It's a no-brainer that Wilson Fisk/Kingpin is Daredevil's greatest supervillain. He's a character initially created for Spider-Man and later shared between the two, though the argument could be made, across comics and now the show, that he's a more compelling antagonist to Daredevil. However, immediately after Kingpin surely comes Bullseye.
He's a terrifyingly-efficient assassin with anything he can get his hands on--basically a deadlier, more maniacal Deadshot--coupled with being a certified sociopath and has made Matt's life hell--pun intended. Historically, he's killed his two greatest romantic loves in Karen Page and even Elektra while carrying a sadistic vendetta/rivalry with Daredevil. For these reasons alone he needs to be the main antagonist in a satisfying resolution to his arc in a fourth season.
Elektra: Corpses Don't Always Equal Death
Comic book fans know that, for better and worse, a dead body doesn't always mean an actual 'death.' Many major characters have been killed throughout comic history: Batman, Superman, Jason Todd (Robin #2), Spider-Man, Captain America, etc. That includes Elektra.
In comics, as mentioned, she's murdered by Bullseye but returns, and her alleged death(s) in Daredevil are similar, with the latter being more ambiguous. Her body mysteriously vanished at the finale of The Defenders and given the aforementioned significance of Elektra as a character and in relation to Daredevil himself, it deserves resolving by bringing her back.
Bullseye: A Season 5 Set-Up
Another potentially excellent opportunity is that writers can flesh him out and give him a more expansive arc over two more seasons, thus setting up the major conflict of a fifth season. Bullseye's certainly a Daredevil-centric supervillain and has plenty against him even in the show, though the series suggests the bulk of his vendetta is against Kingpin.
Daredevil gave him a rude awakening by forcing him to see that Kingpin has been his puppet master, revealing Julie's assassination. Season four could have Poindexter return as an even more vicious, vindictive Bullseye set loose largely on Daredevil first, then the finale could prop up an expanded version of the Daredevil vs. Bullseye vs. Kingpin cathartic climax of season three for S5.
Elektra: Development For Her Season 5 Arc
Similarly to the previous Bullseye point, Elektra could be argued as deserving a focused role in a fourth season as she could greatly benefit from more extensive character development. Specifically, this would be for the reason of having her be involved in a hypothetical fifth season where we see Bullseye in a climactic return.
Season four could serve as further character exploration of both Daredevil and Elektra while facing a different threat, then we see the biggest rewards of that in a 'series finale' in the form of season five that pits them both against an even-more unhinged Poindexter/Bullseye.
Bullseye: Avoid Another Anticlimactic Death
The biggest hindrance Elodie Yung's Elektra faced narratively is that her death felt anticlimactic. She, unfortunately, was killed rather abruptly, though the reason for that was clearly because it needed to lead into The Defenders, which was an issue. If Elektra is written into a season four but no further than that, the issue of another death will potentially present itself.
Her death is iconic in Daredevil lore, but the show overplayed its 'death' card(s). Having Bullseye kill her will make it harder to write successfully and more likely to feel anti-climactic and lazily-written rather than emotionally resonant. And, quite frankly, Elektra, Matt, and Elodie Yung deserve better.
Elektra: No More Narrative Baggage
A refreshing prospect of having Elektra be the season four focus is because a hypothetical series revival could no longer be saddled with the burden of setting up another major crossover event. Crossovers can be fun and pulled off well, like Avengers, Infinity War and Endgame showed (generally), but they're quickly becoming played out and oversaturated, resulting in solo stories suffering narratively.
Season four could let Yung and her Elektra have more agency over herself and go more in-depth, rather than rushing a big team-up. Season two did well in spite of those hindrances, but this could be a breath of fresh air for everyone involved.
Bullseye: Guardian Devil
Bullseye's potential season-four return could bring influence of another iconic, though controversial, Daredevil comic book arc. While the sadistic marksman murdered both Matt's love interests, one not explored was the death of Karen Page in Guardian Devil. Season three gave a thrilling, tense easter egg and homage to her death scene when Bullseye almost killed her in a church.
The comic book was in part controversial due to Karen's cruel life used as a plot device for the story's resolution, Daredevil's triumph, and the continued trope of his life being often awful, but the writing/acting talent could pull off an effective death for her if the team is confident they can write it well, especially since she got more depth than Elektra, being a series regular and all.
Elektra: Both In A Two-Season Arc
Alternatively, both Elektra and Bullseye can be introduced in a fourth season and start a major, two-part arc spanning a fourth and fifth. This would allow for more deserved time for Elektra being an important focus and give both her and Matt a worthy foe/threat to deal with.
The second part of this arc would come in a season five where the three of them have a continued battle, plus the terrifying added problem of dealing with Vincent D'Onofrio's definitive Kingpin. This could add an even more cathartic dynamic than season three - and potentially four - by having Daredevil & Elektra vs. Bullseye vs. Kingpin over 13 episodes. Elektra shouldn't be killed for the reasons mentioned, but the tension would not be dampened.