We don't have any concrete information on this right now, but I think it's an interesting topic to sorta theorycraft about.
Demacia's new thing is that they really, really hate magic. Hextech does, arguably, contain magic, if you want to count the Brackern soul gems as being magical in nature, so that would probably make them ill-willed towards Piltover. However, if we stretch the definition of "magic" too far, it starts to reach the point where Demacia cannot like any city-state but themselves. While that is...actually fairly likely, given how insular they are, it's more fun to go off of other topics and give the magic thing the benefit of the doubt.
Piltover is located as a major trade hub. The territories surrounding it are slowly being assimilated into the Noxian empire. Demacia's other thing is that they really, really hate Noxus. The Noxians have already tried making aggressive moves towards Piltover, what with Tamara being sent in as a spy, and probably not the only one. Piltover is also armed to the teeth with high-power weapons and chem-tech, meaning that they might be the only nation that stands a decent chance at fending off a straight-up Noxian invasion. Even Swain seems to recognize this, considering the stealth he's employing with the warmasons. So Demacia would probably greatly appreciate that. I could easily see them being allied, but a little weirded out by the cultural differences.
There are a lot of directions this could go, and I think it mostly depends on "How open to interaction are the Demacians?"