Keats x Ellen
Folklore is a PS3 Exclusive video game, third-person, release on 2007. Both characters on the cover is playable. I didn't play the game, but I see gameplay on yt. I fall in love with the story and aesthetic, although the gameplay might not be the best and might seem like a chore almost. All the characters design in this is really cool and I like both main character.
minor spoiler:
Personal opinion, but I think there's a possibility of romance between them.
Twilight x Yor
Yes this is action manga (you could argue its a slice of life because of the lack of action, regardless its still action and its amazing when the action appears). This pairing is what I'm talking about when I said shipping. I ship Twilight and Yor so much. Its impossible for this not to be romance. Its just that currently their relationship is slowly developing and its an extremely slow romance.
Randel x Alice (not romance per-se. You'll know what I mean if you reach vol 20)
Note: As the story progress, it primarily focus on politic,war,debate,philosophy,metaphors (start around volume 13+). So the MCs rarely appears in these volumes and many new characters introduced,though soon the MCs both will appear again.personally i think the manga is almost at the end,but its unfortunate that it has been on hiatus for years now.When its on hiatus,it usually mean discontinued,who knows.
Akiyama x Nao
This manga is one of my favourite of all time manga. The story is super good from beginning till
the end. And one of the reason that I like it also because I want to see progression on Akiyama x Nao relationship. This is one of those where the romance is so subtle that you think its not there, and the problem lies on Akiyama.
I don't remember if there's action or not.
I know this list is for action only, but if you're looking for subtle romance, shipping them mcs, then you can try this. I guess its horror, thought its not scary at all. More like investigation detective kind of thing.