Just so we're all on the same page - the 3 Love Interests are Kasula, Glade and Lorelei. All routes are monogamous, except for one. I made a small change to my plans which haven't been reflected yet in the FAQ. There's now a throuple route.
To answer your pleas, I'm sorry... but there's absolutely no chance a side character becomes a Love Interest. Reason being they simply don't have enough screen-time to warrant a path, and even if they did, I wouldn't want to have more LIs. And because they aren't LIs, they're not getting any lewd scenes because I want to focus my efforts on the actual LIs.
Alas, I have planned for Tides to be the first AVN in a trilogy. There are a few side characters in Tides who show up as important characters later on in the trilogy. Most notably, Zelene is a LI in 'Part 2' while Elva is a LI in 'Part 3'. So if you wanted to romance those two characters, then you're in luck, but you're going to have to be patient, as it's going to take some time to finish Tides and then produce those AVNs.
On a separate note, I also want to take some time to address the throuple path.
When I announced it, I decided to keep the details a secret, with hopes that it would draw out some suspense, but now I realise that perhaps this wasn't the best move from my end. So with that being said, here's some information on the throuple path and how to get it...