Oh boy, oh boy, oh boy, where do I start?!
Lem
His full name is Ajuna Lem, he is the young boy who died in Ekko's comic. He also appears on a card in Legends of Runeterra, called Sumpsnipe Scavenger.
https://cdn1.sixmorevodka.com/wp-content/uploads/2019/10/08000453/SUMPSNIPE-SCAVENGER.jpg?webp=0
This boy, who crafted protheses for the injured animals in Zaun.
Ever since they reworked Urgot, we don't have the "Frankenstein's monster" trope in the game anymore, a dead corpse brought back to life through science. It was the right call for Urgot, since he had too many similarities with Sion. Both were Noxian soldiers who got resurrected to keep serving Noxus on the battlefield.
Ajuna Lem could return from the dead by using a mixture of Hextech and Chemtech. But he is not the sweet innocent child he used to be anymore. I mean, he got murdered by that Piltovan, now he is an undead monstrosity. That surely would change a person.
I could imagine that he still augments animals.... but in a more sinister way. Instead of helping injured creatures, he could now start augmenting dead animals in a similar way he is augmented himself. Meaning that Ajuna Lem could have his own little pack of chemtech terrors to haunt Piltover with.
The interactions with Ekko would be amazing. Ekko regrets that he was not able to save Ajuna back then, now what would it do to Ekko emotionally if his former friend returned and became that thing. Ajuna is, btw, a name used by people who were close to him, while people who didn't know him well called him Lem. Hence why I would call him Lem in the game and Ekko should also refer to him as Lem from that point on.... because this creature is not the friend he used to know anymore.
Baroness Voss
We are clearly lacking a chembaron champion in the game.
We hear quite a bit about chembarons in the lore and have a bunch of champions who are out there to kill them, like:
It is time for a chembaron to fight back.
I think Baroness Voss, who appears in Urgot's background story, would fit well. She would have a motivation to fight herself instead of just sitting back and commanding some henchmen. Urgot has ruined her, he took over the dredge and made her run away in shame. I think she would want revenge for this and that it became a personal affair for her.
Also she is shown as quite sadistic in the lore, so I think an agile assassin playstyle could fit her well.
Roe
Appears in the story "Child of Zaun". She is supposed to be dead, but I could imagine that Vi simply thought she was dead, but she actually survived Urgot's trial.
Of course, the poison still should have damaged her alot. So I think that she would have to augment herself with chemtech to stay alive.
She has a good motivation for lore. She admires Urgot as their saviour, so maybe she could become the leader of a group in Zaun who wants to set him free from prison again (since it seems in Warriors that he got captured again).
Maybe they can draw some inspiration from the abandoned design for Husk, meaning that she could have insectoid features in her chemtech thematic. Especially since she admires Urgot and he got symbolised with a spider by the Sons of Ur, I think this would fit well thematically. Maybe the members of her squad could all be designed after different kinds of insects.
What would actually be great: If she would have an upgradable item for her passive in the inventory. But instead of having a single path like Viktor, who then upgrades all his skills 1 by 1, imagine if that item would represent her chemtech augments and it would branch.
So basically,she would have the choice between 3 different augmentations, which also change her design based on different insects!
Maybe a mobility based augmentation with wasp design, one for sustain based on a mosquito, and another for more damage looking like a mantis?
Member of the Glorious Evolved
Mentioned in Viktor's lore, there is a cult that has formed around his idea of the Glorious Evolution. They are calling themselves the "Glorious Evolved". But instead of seeing Viktor as the man he is, a man of science who just wants to help people with his inventions, they started to worship him and seeing his gifts as some kind of "miracle" instead of science. Also they are worshipping Camille as the "Grey Lady" in the same way.
We don't know more about this cult, about their religious practices for example. But I came up with a concept that I would enjoy for them.
I would make their religion inspired by voodoo, I came up with that idea looking (once again) at the concepts from sixmorevodka for Legends of Runeterra.
https://imgur.com/dqY96bL
This is one of the concept arts they made for the game. The character here seems inspired by Baron Samedi, a loa from voodoo religion who is oftenly pictured as a black man with a top hat and a skull drawn on his face.
I would go in a similar direction for the Glorious Evolved, make their culture inspired by voodoo and let them worship Camille and Viktor (and possibly also Urgot, Warwick, and Orianna) as their loas.
Another part of voodoo culture are zombies. Which could give the Glorious Evolved a story arc that becomes darker over time. Maybe they are trying to use Viktor's technology at some point to resurrect dead bodies? This could possibly be a background for the concept of Ajuna Lem I pitched here as well.
Then we could also get one of these fanatics, maybe a highpriest of the cult, as a possible champion.
Midenstokke
Another candidate for a chembaron champion, but this time with the approach of the evil mastermind in the background.
He is only mentioned in Legends of Runeterra, we got a card for his henchmen:
https://cdn1.sixmorevodka.com/wp-content/uploads/2019/10/08000455/MIDDENSTOKE-HENCHMEN.jpg?webp=0
If he is the kind who sits behind and controls, he should probably become a support champion.
Baroness Corina Veraza
And another chembaroness. She appears in Legends of Runeterra.
https://cdn1.sixmorevodka.com/wp-content/uploads/2019/10/08000501/CORINA-VERAZA.jpg?webp=0
What makes her interesting is that she is specialised in bio-engineering, as it seems. Which is something that we don't currently have in the game.
Singed does bio-engineering in the lore (for further information, ask Warwick), but he doesn't show it in the game.
She could be an interesting mage, maybe even with gameplay interactions with the plants in the game!
Also, she gives us another champion with an alchemy theme. Singed doesn't really represent alchemy that well either, in my opinion, so I would enjoy another one. And her skill called "Magnum Opus" (which is an alchemy reference, btw) could become a great ultimate, I guess.
Member of Clan Arvino / Cadwalder / Giopara / Holloran / Medarda / Morichi / Torek
There are many clans with power in Piltover, yet the only clan represented in the game is Ferros:

Note: I don't necessarily mean that this champion has to be the leader of one of these houses. It can also be a servant.
For example, when they were creating Sett, Riot pitched the idea for a robot-butler. That they thought about adding a servant here was really interesting for me.
One idea I would personally enjoy: Adding a maid champion. A young woman who got trained to serve the leader of her clan in Piltover, doing absolutely everything to guarantee the success of her master, including murder. Could also finally give us a more sinister Piltovan. Every Piltovan champion is either good or neutral, we don't have a single obviously evil Piltovan in the game right now. I would like to change that.
These are my ideas so far, I might add more if I come up with more stuff in the next days.