Fire Emblem is getting a lot of love this time. Really happy to see this stage return. Lots of FE music incoming then. Hell yes!
Assuming Castle Siege keeps the music selection from Brawl thanks to My Music, we can expect a whole bunch of new songs for the Coliseum stage.
Id (Purpose) will likely play on Coliseum. The Devoted is confirmed on the Coliseum too. I'd also like to hear a new version of the Fire Emblem theme.
The two other FE songs on the 3DS version (Fire Emblem (Melee) and With Mila's Divine Protection) will very likely stay on Castle Siege.
Should Castle Siege keep the same song selection from Brawl, it's interesting that Eternal Bond (AKA Ike's Theme) and The Devoted (the battle theme for the Greil Mercenaries in Radiant Dawn) are on separate stages.
I also would really like to hear the full version of Id (Purpose) on the Wii U instead of the shortened one on the 3DS game. I can understand why they cut the song: likely memory issues, but the Wii U version has no excuse.
Now to hope for other franchises to get the same treatment.
Kid Icarus--oh wait.
Anyway, I think the stage selection on the Wii U version will be bigger and more diverse. The 3DS version has 34 stages (I think), which is between Melee and Brawl's numbers (29 and 41, respectively), so I could see the Wii U version surpassing 50 stages.
I can see some past stages on the 3DS version appearing on the Wii U version as well.