More ways to customize your heroes

A nice change would be either having the BG on your character screen be area-specific, or having a couple you can choose from/unlock.

Not exactly customizable, but I'd love for certain character to affect the battle theme

Like, if you put the Black Knight in front of your team, the battle theme for everyone changes to 'Against the Black Knight'

You mean change clothing colours like how they did in Smash Bros?

Then yeah, I'd like that. Seeing Marth in white armour would be amazing.

I always thought it would be cool to be able to choose what art of a hero is their resting art. For instance, every hero has 4 unique pictures to them, but its always the first one that is used when viewing their build, being able to set any of the 4 to act as their 'home' image would be cool.

Another small thing would to be able to review the successful defenses in arena, that way you can see what parts of your team are working and what should be replaced or improved.

I also think a cool thing to help every unit would be that you could give a stat boost to a single stat by sacrificing another unit. for instance; lets say I wanted to boost my B!Tharja's speed, my B!Tharja has a base speed of 35, I could sacrifice my +spd Karla (44 base speed) to make B!Tharja's base speed 44. This could only be used on a single stat and would need a 5* sacrifice. This boost would not count towards the units BST.

I want unlockable "skins" for characters, similar to accessories but with more design to them and exclusive to specific characters. It'd be cool if they even gave really small stat boosts too.

I think it'd be cool if you could change the appearance of your weapon. Some weapons look cooler than others, and it would be cool to be able to change the appearance, but not the effects. Since this would make things a bit misleading it could be the user side only. Other than that, the color pallete changes could be cool.