"Raiden to Leviathan?" and "New Battlefield?"

Raiden to Leviathan should be possible. I can help you figure out how to copy the animation from FF6 Advance. I would recommend starting with Flood first and then try the Leviathan summon.

Adding a new battle background should be pretty easy, especially if it's just a palette swap. You would need to change the battle background properties at E7/0000. The format of the data is on the wiki. There is also a table at ED/8E5B that determines the dance index associated with each battle background.