I'd love the ability to hear trainers bikes when running with them, as for me personally the noise of being followed adds a whole new layer to lapping in GPB, and would get newer players used to it before they jump online.
Also the ability to see trainers in replays would be awesome, it would allow for mod packs such as MotoGP packs to be showcased all at the same time on track, and if its combined withe the sounds it could be awesome to see and hear. Especially if it can load separate paints