It could be done the same way as Mythic Paths. "When you begin Adventure 4, choose a role card. For the rest of the AP, when you play a scenario in Adventure 4 or higher, the card you chose is your role card. After you choose, gain a power feat."
As much as I loved being able to only take two power feats on my base card for some characters in Season 0, I think the best way to handle it with the tier system is gaining your role and a power feat when you advance to tier 4. With the rule you suggested, there can be some really funky game-working attempts to try to only having to take two power feats on your base character card. Just advance to Tier 3. Play one scenario to get your skill feat. Wait to play an Adventure 4 scenario to get your role and a power feat on it and then complete the scenario to get another power feat from your tier also applied to your role. Then you go back and finish the Adventure 3 scenarios to finally advance to Tier 4 and gain your die bump and whatever rewards are available for Adventure 3 completion. With the help of your Mythic Path, you probably won't be missing that Tier 3 power feat while you finish up Adventure 3, especially if it wasn't something you wanted for your character anyway.
The Tier System is meant to make it easier to get people to play in sessions that will let their characters advance. If we use rules that reward people for being more selective of the scenarios that they play, it is counter to the goal of the Tier System to begin with. So yeah, link roles to Tier advancement, please.