Earthbound64
Banned
64DD F-Zero is the obvious choice.
I would buy it for that alone.
N64DD games like Mario Artist and F-Zero Expansion Kit
Localized versions of Animal Crossing and Doshin the Giant (based on GC releases).
Zelda Master Quest
Banjo Kazooie and Tooie with Stop n Swop
The others are a bit pipe dreamy but Master Quest and maybe the DD games I can see since they already exist.
Sin and Punishment would be an easy but cool one too.
Edit - Ooh, here'd be a fun one...
Compatibility with a GameBoy Classic for Pokemon Stadium.
Of course, this would be a dream come true.