As far as couples costumes go, queer couples usually get the short end of the stick. More often than not, costumes, especially those based on pop culture, tend to be gender specific for straight couples. However, I was able to wrack my brain and come up with a few ideas for LGBTQ+ couples.
1. Harley Quinn and Poison Ivy (DC Comics)
Harley and Ivy are arguably a classic for lesbian couples and are especially great for outgoing ladies. For Harley, any combo of black, red and pigtails will do the trick, especially accompanied with a New York accent and baseball bat or hammer. For Poison Ivy, any form fitting dark green outfit will work, although corsets are a favorite among cosplayers.
2. Cecil Palmer and Carlos The Scientist (Welcome to Nightvale)
Since Carlos and Cecil don't have any appearance described in canon, this can be left up to interpretation a bit. However, Cecil is generally viewed as well dressed, wearing dark clothes, vests, and often shades of purple. Carlos, on the other hand, is usually drawn in fanart as wearing jeans, flannels, and of course a lab coat.
3. Ruby and Sapphire (Steven Universe)
Red workout gear and a blue dress are all you really need for this costume, but feel free to add hair dye and body paint if you're super committed.
4. Alexander Hamilton and John Laurens (Hamilton)
5. Princess Bubblegum and Marceline (Adventure Time)
For a girly and grungy couple, Marceline and Bubblegum will be easy to replicate. All you need is a black tank top, jeans and boots, with an optional guitar, for Marceline and a pink dress and crown for Princess Bubblegum.
6. Kurt Hummel and Blaine Anderson (Glee)
7. Prisoners from Orange Is the New Black
This can be a super cheap costume if you're on a budget. Just get some beige scrubs and name tags. Viola, you're a prisoner from Litchfield.
8. Walter White and Jesse Pinkman (Breaking Bad)
Hazmat suits, gas masks and goggles are surprisingly easy to find at Halloween specialty stores. For added authenticity, offer friends rock candy in small baggies - but maybe don't hand it out to kids.