Does anyone have any idea if there will be a reward like a banner or something for having taken part in the insider program?
I don’t believe there are any such rewards being given to Halo Insider participants. I know that Insiders who were invited to the Xbox MCC Reach flight and played 5 or more games during the stress test received an MCC Insider nameplate, and those who played 12 or more games during the stress test received an additional nameplate, as well. I have not heard of any other Insider rewards, though.