Hi there, Mischiefmaker - thank you so much for writing for me! This letter has a lot (maybe too many???) ideas and suggestions to get you started, but I'm going to adore whatever you write even if you ignore all of the below. This is one of my favorite exchanges, both as a writer and as a reader, and I'm going to be impossible to disappoint.
I'm down with any rating from G to NC-17, although porn for porn's sake is usually not my thing. My favorite tropes include secret identities and identity play, female friendships, battle couples, epistolary stories, pining (especially when it's for reasons related to duty rather than poor communication), happy endings, sad endings, earned endings, cats, crack, atonement arcs, characters who are stiff with honor, cosmic horror, and competence. Het, femslash, and slash are all green lights.
Elements I dislike are animal abuse, infidelity, rape, and self-harm. That said, I do like dark fic, too, so if one of these themes is a critical element in what you're writing, all I ask is that you not make it graphic.
I'm not a big fan of post-canon fluff for this pairing, nor am I particularly into Loki redemption stories that aren't set centuries in the future or that don't involve big, drastic sacrifices on Loki's part. My favorite dynamic for Loki and Sif is that of two people who at one time deeply understood each other, had a shared sympathy as outsiders, certainly respect and maybe still love one another, but who will never again trust each other. They've gone their separate ways, and neither of them is willing to give ground. On the other hand, if you want to stick them in a situation where they're forced to work together despite intense anger and wariness... go for it!
I'm intrigued by the idea of Sif visiting Loki while he's imprisoned in Asgard (can we ever have enough stories about this???). I'd also love anything with Sif interacting with Frigga or Jane (I lovelovelove Jane Foster and think she and Sif would get a kick out of each other given enough time together), Sif being the one to hunt down Loki after he makes an escape, Loki being irredeemable, Sif being stiff with honor and devotion to duty, or either one of them being determined to define their lives on their own terms despite opposition and especially if it forces them to admit their relationship can't survive.
Bonus points for Sif getting the upper hand on Loki, double bonus for guilty secret post-betrayal assignations, triple bonus for Sif and Heimdall sibling shenanigans. If you have Sif run into Gamora in the process, I will positively swoon.
If you'd like to dig around more, I'm active on AO3
(anon messages welcome!). Oops, and just as a note - I'm familiar with a decent swath of comics canon and with most of the MCU; I've really only retained the AoS episodes involving Sif, but everything else is fair game.
Thank you again for all of your time and effort!! :)