So in the secret ending there is a guy who is most likely Sephiroth's "genetic" brother and he carries Weiss off into the sky. Kinda obvious that this is the G fellow (Genesis right?)

Perhaps he wants to revive Sephiroth using Weiss and Jenova cells much like what Kadaj did.

Do you think the next installment will be another movie or a game? RPG? Shooter?

I know Crisis Core talks about Genesis but CC is in the past , not in the present, so it won't give many answers.