For some reason, I can see them adding Diddy Kong and Petey Piranha to fill some position for some reason; they've already got King Boo. And yeah, nameless, generic characters is kinda' filler-ish, they should just do what was done with Krunch, instead of a generic Kritter, they made a new character.
But if I had to choose, I'd go with the Koopa Troopa, its been playable in several other games and has appeared throughout the Mario Party series as a quasi-character.
Koopa Troopa's more sensible, but I like Shy Guys too much to not vote for them. Also, playable, nameless members of a species don't bother me; I'd almost rather have a random Kritter than Krunch, who just popped out of nowhere for Diddy Kong Racing. (Hmm... Would we want some newbie name "Waluigi" to pop out of nowhere, or have a random human named Human appear? There's a hard question... :P )