Most of the fandom t-shirts that I have are a little baggy since they tend to only be sold in men’s sizes, so I usually wear them with slim trousers or jeans. I also usually wear them with a blazer or feminine-looking cardigan to make the outfit look more styled. There are times, however, when I really dig the androgynous look and let that inspire my fandom outfits (see Star Trek photo at the bottom).
Black Milk has several lines of fandom clothing that I absolutely love. I especially love these leggings from Lord of the Rings and Harry Potter. Because they’re printed and very busy, I would keep the top very simple- either black or a plain dark colour for the Lord of the Rings leggings. For the Harry Potter ones, because they’re black and white, you could just do any plain, coloured shirt.
Disney
I think the Disney fandom deserves honourable mention. I believe you're never too old to want to be a princess, and I am one 22 year old that is not afraid to show it (my mom thinks I'm crazy for being so in love with the Disney princesses). I bought this top from Hot Topic and it's so fun to style because of all the colours in it, but that day I opted for simple black and white. Some people may feel like a little kid again, walking around in these kinds of shirts, but I think if you style it properly, you can look as mature and grown-up as you want to.
Skirt: Garage
Blazer: Tristan
Bag: Longchamp
Top: Jason Palmer Studios via Fanexpo
Cardigan: Tristan
Pants: Joe Fresh
Shoes: Zara
Top: HMV
Blazer: Costa Blanca
Pants: Joe Fresh
Necklace: Target
Top: Disney World
Blazer: Attitude Jay Manuel
Belt: Old Navy
Top: Forever 21
Sweater: Forever 21
Shorts: Urban Planet
Top: The Disney Store
Blouse: Joe Fresh
Jeans: H&M
Shoes: Old Navy
Top: HMV
Jacket: Talula via Aritzia
Hat: H&M
Top: H&M
Skirt: Hot Topic
Blazer: Urban Outfitters
Shoes: Keds