This is a very long letter with lots of stuff, but DO NOT PANIC. I'm actually fairly easy to please; I am very rarely disappointed with a ficathon story. I write long and detailed Dear Author letters because I find such things helpful when I'm writing for other people; if you are like me, here you go! If your style is different and a detailed letter makes you feel hemmed-in, feel free to do what works for you.
The most important thing for me in a fic is that the characters are well-written and recognizably themselves. Even when I don't like a character, I don't go in for character-bashing. If nothing else, if the rest of this letter is too much or my kinks just don't fit yours, just concentrate on writing a good story with everyone in character and good spelling and grammar and I will almost certainly love what you come up with.
One thing: I do have an embarrassment squick, which makes humor kind of hit-or-miss sometimes. The kind of humor where someone does something embarrassing and the audience is laughing at them makes me uncomfortable because I identify too much with the person getting laughed at, so instead of being funny it is squicky. On the other hand, the kind of humor where the audience is laughing with the characters I really enjoy.
If you want to make me the happiest recipient ever, here are some other things to keep in mind (not all of which will apply to all fandoms):
Dottie Underwood. I am really interested in Dottie. What was the Red Room training like and how does it still affect her? What was she doing between seasons 1 & 2? Are there other current Black Widows, and what happened to the other girls she was training with? Is she still a Red Room agent or on her own now, and what's the larger backing to the Red Room (KGB, HYDRA, general Soviet villainy)? How does all this fits into the larger MCU framework? Does she ever join up with the nascent HYDRA in the SSR? Does her fascination with Peggy ever wax and wane? Does she ever work with the Winter Soldier?
Howard Stark/Whitney Frost. Howard Stark/Whitney Frost: what changes, what stays the same? If she no longer has to kill people to get to do the science she *really* wants to do, how does that change her? (I would think that depends on when they meet, how much sexism/creepy guys she's had to put up with to get where she is. Like, young!Whitney just starting out wants nothing more than to do science, whereas post!season2!Whitney. The ep with her backstory made me wonder if she was a victim of some sort of sexual abuse, both with her "uncle" and later that first agent, but I don't insist on that interpretation. (Goes cold and still around him, "I don't like him" but won't say why--could easily be a child testing the waters, what is acceptable to say about the abuse she's experienced.) Howard's a dick, but his reaction to competent women seems to be "awesome, let's work together, can we create something," which Whitney desperately needs. If you do something post!season2, though, it shouldn't be an easy relationship or redemption. If you do a fanvid for Whitney, The Rules of the Road by Cy Coleman would be a great song for her. (I like Rosemary Clooney's version best, but I couldn't find it on Youtube--it's a little more upbeat than the version linked. Lena Horne also did an awesome recording of it.)
Peggy Carter/Gabe Jones. This is the comics pairing, and I read a couple of AWESOME Peggy/Gabe fics early on, and really imprinted on the pairing. I have been very "meh" about both the pairing options we were offered in season 2. Where was Gabe during all this? What is he doing, post War? Does meeting up with Wilkes give Carter a nudge to go look him up again, or do they meet again on a case, or have they been corresponding this whole time? What are some of the racial/sexist issues they're going to face as a couple in the late 40s?
Rose Roberts. She loves California! She loves her job! She's such a cheerfully competent character! I WANT LOTS OF GREAT FIC ABOUT HER SAVING THE DAY THROUGH STEREOTYPICAL MIDDLE-AGED FAT WOMEN SKILLS. Like, she ALWAYS has just what you need in her large purse, and it serves as a great blunt object in a fight because it is SO HEAVY AND FULL OF STUFF that when she whacks you with it, you go DOWN. I don't want anything depressing. I want her to be happy and fulfilled and to have FUN.
Peggy Carter. Peggy is so much fun, and I love every aspect of her. I particularly would enjoy casefic, but I would also enjoy her hanging out with friends (Angie, Edwin and Ana, Rose, any of the Howling Commandoes are the ones I'd be most happy about. Howard, Daniel, and Dr. Wilkes I wouldn't mind. Jack Thompson I don't like.) And I would also welcome Peggy/Steve, set during the war, flashbacks, AU where he gets rescued, whatever. If you were going to do a fanvid of her, Pistol Packin' Mama by Bing Crosby and the Andrews Sisters would be AWESOME. For Peggy/Steve, the obvious song for a fanvid would be It's Been A Long, Long Time (the song that Nick Fury put on in Steve's apartment in Cap2, it would also be a perfect Steve/Bucky fanvid) (My favorite cover of that song is the one linked, Harold Arlen's orchestra with Kitty Kalen on vocals).
General songs that would make good fanvids for the show that I would love to receive even if they don't fit the characters/pairings I requested:
It Takes a Woman from Hello, Dolly
Hot Rod Mama by Ramblin Jimmy Dolan
A vid about jingoism, patriotism, and obfuscation could be set to "Motherhood" from Hello, Dolly
I'm not enough into Jarvis to want a fic about him, but I would LOVE a Jarvis vid to Stiff Upper Lip by the Gershwins (recorded by so many artists, you have a lot of selection.)
Imagine My Frustration is Ella Fitzgerald belting out her frustrations about being overlooked. Could work for Peggy, Dottie, Whitney, Wilkes, any or all of them.