The support characters should be fixed so they work better.
New characters (support characters in parenthesis)
Franky (Mozu/Kiwi, Kokoro, Zambai)
Paulie (Iceberg, Lulu, Tilestone)
Spandam (Wanze, Nero, Funkfried)
Lucci (upgrade to Leopard form for a limited time, getting stronger)
Kaku (Blueno, Kumadori)
Califa (Jyabura, Fukurou)
Sogeking (EL giants) (he'd be like Luffy Afro was, just an alt.costume and the same except for secret move, and in this case his support would be different)
As far as full amount of characters, I have a list of 40 and their supports that I'd love to see as a full roster. 40 is asking alot though, but still it'd be awesome. I'll put it in spoiler tags so it doesn't take up too much space. Maybe one day they'll make a special version of the game with characters from throughout the whole series.
! Luffy (Coby, Laboon/Crocus, Gaimon)
Zoro (Johnny, Yosaku, sword salesman guy)
Nami (Genzo, Eyelash, Gan Fall)
Usopp (Usopp pirates as one support, Dorry, Broggie) (EL giants as Sogeking)
Sanji (Zeff, Pattie, Conis)
Chopper (Kureha, Dalton, Gan Fall)
Robin (Hana-hana powers v.1, hanahana v.2, hanahana v.3)
Vivi (Karoo, Kohza, Igaram)
Buggy (Richie/Mohji, Kabaji, Alvida)
Kuro (Siam, butchie, Django)
Krieg (Ghin, Pearl)
Arlong (Hatchi, Chuu, Karoobi)
Smoker (Tashigi, Navy soldier)
Tashigi (Navy soldier, sword salesman)
Pell (Chaka, Igaram, Alabasta guardians)
Wapol (ChessMaarimo, hippo thing, 20 doctors)
Mr. 1 (Ms. Doublefinger)
Mr. 2 Bon Clay (Lassou, Mr. 4, Ms. Merry Christmas)
Mr. 3 (Ms. Goldenweek, Mr.5, Ms. Valentine's day)
Crocodile (Bananadile, The unluckies)
Hina (Django, Fullbody)
Zeff (Pattie)
Bellamy (Sarquiss)
Cricket (Masira, Shojo, Southbird)
Wiper (Laki, Kamakiri, Braham)
Gan Fall (Conis, Pierre)
Enel (60,000,000 Volt Julungul)
Ohm (Shura, Gedatz, Holy)
Satori (Kotori, Hotori, Yama)
Ace (fire boat)
Shanks (Beckman, Lucky Roux, Yasopp)
Foxy (Porche, Hamburg)
Aokiji (ice bike)
Franky (Mozu/Kiwi, Kokoro, Zambai)
Paulie (Iceburg, Lulu, Tilestone)
Spandam (Wanze, Nero, Funkfried)
Lucci (Leopard form makes him stronger)
Kaku (Blueno, Kumadori)
Califa (Jyabura, Fukurou)
Mihawk (Greatest Impact)