I think the most important chars to add next are Tien and Yamcha... cause they came up very early in the story of db and stayed until the end... they changed gokus fate more than once, and goku is beside bulma the one who started the saga
okay, pikkon is strong but his role in the story is more a joke, and he never shows up in the original manga
even chaozu is more important than pikkon, but i think chaozu is to weak and he never got fans, so he dissappeared from the story...
add tien and yamcha, then we have the whole group Z complete
after this the next important where adult gohan, goten, chibi trunks (gotenks)....
and in later releases there could be chars like Pikkon, C16-C18, Kaioshin or grunts like radditz, nappa, ginyu force....
but first of all they should complete the main group Z at the date of the cell saga + buu/goten/chibi trunks(gotenks - cause they have many fans, and imo they are very important for the story)
and balance this chars well.... and then, later, they should slowly add new "siderole" chars add to the game
like i said above