Did some analysis of new characters in each series and based on this came up with my personal selection for SF6s roster.
You can see on average there were 14 new characters in each series. Based on a roster of 44 (both Ultra SF4 and SF5 will have this amount), less 14 new characters. I have divided the 30 remaining characters as follows :
SF1 12 15% 5
SF2 14 17% 5
SFA 14 17% 5
SF3 16 20% 6
SF4 11 13% 4
SF5 15 18% 5
Total 82 30
SFEX 18 (for reference)
SF1 Ryu, Retsu, Lee, Geki, Adon
SF2 Chunli, Guile, Dhalsim, Deejay, Bison
SFA Cody, Rose, Rolento, Karin, Maki
SF3 Dudey, Alex, Hugo, Elena, Makoto, Oro
SF4 Hakan, C.Viper, Juri, Rufus
SF5 Rashid, Menat, FANG, Zeku, Laura
14 New Characters from following Countries/Regions
Australia
Phillippines
South Africa
Saharan Africa
Western Africa
Central America
South America 1
South America 2
Not yet respresented Western Europe (I.e Ireland, Netherlands, Portugal ete)
Another South Asian (Pakistan, Sri Lanka, Bangladesh etc)
Far East Asia (Vietnam, Cambodia, Laos)
Persian (Basically Iran)
Scandinavia
Eastern Europe