Idea #1! Collei & Tighnari & Cyno (!!not a ship!!)
(!!Spoils for the manga!!)
Extra info: I did use Collei's Genshin fandom Wiki page to make the most sense out of this.
Angst, Hurt/Comfort
Something happens to Collei that gives her the worst flashback(s) --Causing her to reawaken Eleazar somehow. (Which disappeared after Greater Lord Rukkhadevata's consciousness was removed from Irminsul. Curing all the Eleazar touched people of Sumeru.)
(Xtra info) When Paimon touched Collei, she freaked out. ('This shock apparently triggered a flare-up of her disease, causing her to be bedridden the next day.' - Gen shin Impact Fandom Collei Wiki page, personality, last paragraph.)
Over time, rather than her being focused as a Trainee Forest Ranger, she becomes clumsy and dazed. (Ex. shaking hands, tripping, unfocused, dropping stuff) Her cheery attitude dims a little.
Tighnari knew something was off with Collei, but when he asked her, she would shrug it off. Tighnari wouldn't ever force her to share her feelings. But he could tell that there were physical (and mental) impacts on her that were negative.
After about, however long, (you can decide) Tighnari had enough. He wasn't going to keep letting Collei destroy herself.
He knew about the occasional flare-ups she would get, and he was mad at himself for not seeing the signs sooner. But when he finally did he had her take a break and stay home.
Tighnari could never leave Collei alone when she was in this state, so he stuck around with her. Eventually getting her to open up and explain what happened. (Ex. What caused the flashback? What was the flashback?)
Tighnari listened to everything Collei said, willing to comfort her.