Hmm if I were to choose which Type-moon characters to add in Melty Blood it would be:
Sensible ones:
Touko Aozaki
Azaka
Dark Elesia
Asagami
Araya
Joke chars:
Inui Arihiko
Seven Heaven
Seo Akira
-Azaka would be an Akiha clone, we already have enough of those.
-Dark Elesia would just be a female Roa (oh wait, he's already immasculine enough to be female as it is).
-Asagami I don't personally feel would translate into a fighter well. All she could do was twist shit up at a distance, and you couldn't even actually see the power itself without mystic eyes. She's not physically adept in the slightest, nor does she have any kind of superhuman abilities. Miyako doesn't make much sense, but at least she can kung fu bitches in the face, you know? Asagami's only power is fucking invisible. =\
-Araya could be pretty good I guess. You could have zombie/puppet summons and some kind of barrier counter shit I guess. He's got physical prowess as well, so you could prolly do something interesting enough with him. He's still sort of boring though, imo.
-Seven...I guess they could do some kind of silly stuff for a joke character, but we really don't need more joke chars. =\
-Seo ^
Touko, SHIKI, Kiri, and Arihiko (not as a joke!) actually have lots of potential though.
-Touko has puppets, ether projected briefcase summons, and rune magic - which could be long range (writing runes in the air with her cig) or close range (writing runes directly on people).
-SHIKI has mad physical prowess (demon strength!) and all the awesome manipulation of his own blood shit.
-Kiri's a badass assassin with big spikes as weapons.
-Arihiko has...DELINQUENT POWER! (totally legit!)