Rivals to lovers is one of my favorite tropes - granted, I like it a little better as "rivals to allies to friends to lovers," but that's such a mouthful. But positive/constructive relationship growth in general is my favorite thing
, and while the trope could lead to destructive relationship growth (rivals become lovers and it blows up in their faces/their relationship really is as dysfunctional as their rivalry would suggest), I don't come across a lot of fics that treat it that way.
To get the full "rivals to allies to friends to lovers" effect usually requires a hefty word count to show the transitioning/developing relationship, but I love me some longfic. And even when it's a more direct rivals to lovers fic, there's potential for longfic. And I can't deny that direct-to-lovers fic makes for more belligerent sexual tension, which I like a lot.
Early on in Psych, for example, Shawn and Lassiter have a relationship that runs the gambit from simply competitive to irritated to outright hostile, but with an edge which lends itself well to rivals to lovers. My favorite of these is probably What Happens In Freedom Stays In Freedom
by Lenore, a really excellent get together/caper fic written for Yuletide 2007. It's got this slightly absurd nature that really suits the show, and has a great Lassiter voice.
Teen Wolf is good for rivals to lovers fic too - though the life-and-death situations makes the term "rival" feel a little inaccurate, the bickering and tension between characters definitely fits the trope. Unmovable & Unstoppable
by maderr is a good Derek/Stiles fic in this respect, though it takes a long
time to move from one state to the other. It's also got an interesting casefic element to it, which I can always appreciate.
I also like fic that fits the anger-to-attraction mood shift of rivals to lovers, even though the characters involved aren't always rivals. Just What is a Karmic-chi Love Thing, Anyway?
, an NCIS fic by catwalksalone, takes an episode where Tony and McGee being stuck on a stakeout has made them petty and quick to anger, and turns their cut-to-commercial scuffle into cut-to-commercial frottage. Cat always has an excellent Tony voice in her fics, and this one is no exception. You do probably want to be familiar with the episode in question, though, since the events serve as a strong framework for the fic.
The Mindy Project is great for het rivals to lovers, small fandom though it is - the titular Mindy has off-and-on antagonistic relationships with so
many people. Her coworkers, the midwives downstairs, total strangers... even some of her canon relationships get antagonistic. when you wish upon a star (or bury some mustard seeds outside your place of work) you look like an insane person (but i'm into that, i guess)
, a Yuletide 2012 fic by Kindness, does a really good job of infusing Mindy's narration with those antagonistic thoughts. It works really well from the first line to the last.
And then there's fic where, even when they become lovers, there's still an antagonistic element to their relationship. (aka: hatesex) The Curves of Your Lips Rewrite History
, a Les Mis fic by zamwessell, does a pretty amazing job of taking characters who canonically don't really do either hate or sex and presenting a convincing hatesex relationship between them. (There's also really nice, gradual growth and realization of feelings going on in this fic, though the characters take their time in acknowledging that the hatesex has become something else.)