I've been reading a lot of fics for those two pairings, and I've noticed a couple of trends. For Xichen/Huaisang, there are a lot of people who believe that NHS would pursue a relationship with LXC during his revenge period, even if the relationship ends after the death of Jin Guangyao at Guanyin temple. Conversely, canon fics for Cheng/Huaisang almost always tend to be post-canon. Even if they're together before the revenge plot, they have NHS break up with JC once he starts focusing on taking down JGY.
I'd like someone to write something about the transition period, as NHS leaves the relationship with LXC and moves on to JC. (If you're ambitious, you could even do one from the beginning, where NHS breaks up with JC to get together with LXC.) You could also go the threesome route, although if so, I'd like the focus to be on NHS, rather than getting LXC and JC together.
Xisang, Sangcheng - Pre vs Post canon - any universe
I'd like someone to write something about the transition period, as NHS leaves the relationship with LXC and moves on to JC. (If you're ambitious, you could even do one from the beginning, where NHS breaks up with JC to get together with LXC.) You could also go the threesome route, although if so, I'd like the focus to be on NHS, rather than getting LXC and JC together.