I would like to share my suggestions as well. I hope these ideas haven't been suggested already.
Add a better track generator along with the usual stages - DiRT 4 had a lot of potential with the track generator. Too bad it wasn't as amazing as I thought. Generated tracks feel the same, there's not much variety. I think that a better algorithm with more variety would make this feature a lot better and it would greatly improve the overall longevity of the game. Some tracks in DiRT Rally and DiRT Rally 2.0 can be memorized fairly well. Randomized tracks would make the experience a lot more challenging in my opinion.
Add at least one more long stage for every location - DiRT Rally and DiRT Rally 2.0 share the same policy of managing tracks: each location has two long stages, these two stages are splitted in half and every variant is driven the normal way and in reverse, making a total of 12 stages per location. Adding one more long stage would add 6 more stages, for a total of 18 which is plenty and it would also make track memorisation harder.
Some cars to add - I noticed that some DiRT Rally cars are missing in DiRT Rally 2.0, I would love to have them back in DR 3.0 especially the 2011 Ford Fiesta RS WRC (which is my favourite rally car 😅), the 2015 Volkswagen Polo R WRC and the 1984 Audi Sport quattro B2.