Well a few months ago I wanted to improve my early-game Reborn experience to continue a run I had. At the same time, I wanted to pick tracks that fit better, sounded better and, most importantly, weren't used in Reju! The last one keep Reborn distinct and unique. So I went ahead and replaced...
-the city's wild pokemon theme with the Johto Trainer Battle Remix (because I felt this is the closest to pokemon in-game music for that kind of thing, repeated battles, and also I love Johto's music)
-the city's trainers theme with the Pokémon Prism Gym Leader Remix (because the next one for trainers is the Emerald Frontier Brains theme, which reminds me of fencing, so I picked a theme that reminded me of swordfights too, more piratic however)
-and the rivals' theme with the Hoenn Gym Leader Remix (because it's amicable enough but has some tension for that Fern guy, it's nice and warm without getting too hot for the occasion, also a really rich theme)
I picked older themes, I think the mesh better with the rest of the soundtrack at that point. Honourable mentions for the rival theme were the Johto Rival Battle Remix (love it, but didn't fit, thanks to our rivals being too tame to make me feel the EFF YOU that Silver did), the Sinnoh Gym Leader Battle Remix (too thought ful for them, not as light-hearted a theme as these three deserved)
I also considered replacing the minor bosses theme but decided against it. I could have tried stuff... but it's really nice too and it also plays in overworld scenes.