I can't really see the XD and Colosseum graphics on 3DS. It would be too small imo. Kind of how people complained on how squished TotA sometimes got? Though I haven't played the 3DS version so I'm going off of what others have said here.
Hm. Well. If the want to make how you view the game itself (because walking through cities and such is different in XD and Colosseum than it is in the main stream games and what are walking from place to place in those games) then ok. :T but keeping it how it is and making it more 3D wouldn't make the humans look right.