We know for sure the theme will be RPG Fantasy + Nightclub but we don't know for sure yet what sort of nightclub it will be. I have made playlists for the two types we are entertaining.
Option 1: A suave nightclub that plays music from the 70s and 80s
Year 3 Project Nightclub + RPG Theme - Vibe 1
Option 2: Something more modern and edgy
Year 3 Project Nightclub + RPG Theme - Vibe 2
Regular battle music has to be composed with special attention to not be annoying as it will be repeated for a big part of the game. It also has to be a track that can loop endlessly as the duration of the battles is fully dictated by the player as different approaches will take a different amount of turns to complete a battle.
Boss battles on the other hand can afford to be more eccentric as they are not heard too often.
In order to reflect them theme of the game and represent both of the genres it mixes it would be interesting to try to incorporate chorus on the song that plays when fighting against boss enemies.
For example if we decided to go with Option 2 described above, modern music would make known the Nightclub side of the theme while the chorus would convey the epicenes so often associated with the tales told in Role Playing Games. The following song is an a good example of how it could sound.
One of the ways we could use to create exciting moments is by having the song playing change depending on what is happening. If a player character is low on hp it would be interesting to have the track playing become more tense by adding another instrument, for example drums.
It should be fairly easy to implement as all we would have to do would be to have two tracks playing simultaneously, one with the regular song at regular volume and other with the drum track playing at 0% volume. When the predefined condition for the song to change happens (a player character being low on hp as described above, for example), the game would raise the volume of the song with the drum track from 0 to 100% and the player would hear both tracks as one.
Adding an extra drum track is just one of many ways that could be used to hype the player, police sirens or having people start cheering would work really well too.
Example with police sirens:
Sudden Death Mario Strikers Charged Music Extended
Example with crowd cheering:
Pokemon Sword & Shield - Gym Leader Battle Music (Cheer)