My favorite game is Legend of the Dragoon and hoo boy.
Character: Dart Feld or Rose. Dart and Rose are two of the main characters of this JRPG that wield the powers of Dragons. These powers were used during the Dragon Campaign to free all creatures born of the divine tree from their cruel rulers known as the Winglies.
Stage: Moon that Never Sets. It's the final world of the game and there is an area where it has the world of Endness in different points in history or the final Boss' stage where a new world is being created.
ATs: Lloyd. Dart's rival and a member of the most dominate race on Endness known as the Winglies. OR we could do the Virage, which were very powerful beings that were used in the dragon campaign to kill the dragoons.
Music: Legend of Dragoon's battle music sadly dont have names so I'll need to link them.
First battle theme;
https://youtu.be/jPpKMcUwVHk
Second battle theme:
https://youtu.be/YNBaGU6BcFA
Forbidden Land battle theme:
https://youtu.be/4S5qrbxGKPc
Boss battle themes (main three + final boss):
https://youtu.be/JfuR2EnHDFc
Imperial Capital Kazas:
https://youtu.be/oWFudUapiIs
Dart's theme:
https://youtu.be/l91rfBdd5SM
Lloyd's theme:
https://youtu.be/o_hxnXabL8U
Rose's theme:
https://youtu.be/fmI_TsxM_Fg
Zieg theme:
https://youtu.be/YPSi_1SKa-w
In terms of what they could bring to the table, Dart and Rose easily fight in both their normal forms and Dragoon forms. These can easily be their movesets.
Rose moveset (base):
https://youtu.be/PBxFBN_Rsy8
Dart moveset (base)
https://youtu.be/NzabuxIhOz8
Their gimmick could be once they accumulate enough SP (through attacking similar to L.Mac) they get to transform into their Dragoon forms where their movesets increase.
Rose moveset (dragoon):
https://youtu.be/9bTL5Z4-Lz8
Dart moveset (dragoon):
https://youtu.be/YhX-zlwOpSA
I got ahead of myself. Point is Dart and Rose would bring a good chunk if they were given shot in smash and they are my true number 1 pick, with 2B being my second