Hi, inkstar,
to AW!
First of all, I'm going to say that I'm very excited to hear about any novel that doesn't include a love interest. I just finished a YA SF with almost no romance, and it felt refreshing to me (Hullmetal Girls). So often, YA plots are sidelined by the love interest, and it can get tiring. Don't get me wrong, there are also great YAs with central romance themes, but it's so novel when someone writes one without.
I'm not entirely sure, though, that you really mean to avoid a love interest. You actually sound as if you want to have one. Is it a love triangle you're trying to avoid?
I worry that you say you can't see how to write a Shadowhunters-like story without a love interest, though, because I worry that you're depending too much on the Shadownhunters/The Mortal Instruments story to build your own story. Your story should be completely different even if you're inspired by TMI/Shadowhunters or use elements you like from that story. As such, it should be easy to make your characters have any sort of relationships you wish.
Some examples of relationships that can be important in the story that aren't romantic:
- Friends
- Family
- Comrades in an organization of some kind
- Reluctant allies working towards the same goal
- Rivals
- Frenemies