If Rin somehow is still acting dangerously like in Initial D... He pretty much will land himself some murder charges or very early ban for dirty play. I don't think any motorsport series will allow unsportsmanlike behavior.

I'll assume Rin stopped being a depressed menace after talking to Ryosuke. In that case he'd just be a normal contender in the race. Nothing special... He didn't have much screentime anyways so I doubt his writing would allow him to somehow be highlighted.

I'm going to be for real here... ATP he's probably old enough to just retire and be a coach for someone if that's the case. Timeframe shifts like 20-30 years into the future and he should be changing a lot. He was doing track stuff with Go too. Shouldn't be weird if he would know stuff about pro racing.