My few cents.
In the past we have seen also a lot of impact of well selected gear ratios in Ypres because the event is very specific but I don't think it will play that much of a role with WRC cars as it had with S2000 cars (there it was very large factor due to the atmospheric engines). In general in Ypres you need often to accelerate all the way from first gear to sixth gear or to keep very high speeds. You don't need to pay that much attention to torque delivery in medium speed corners like in Spain or Corsica where it's all about that.
Reaching higher top speed is also more important than in other asphalt events and in the past especially with S2000 cars it was one important factor too (probably there won't be that big differences among WRC cars).
Traction from the slow corners is crucial too and will be even more with the mud. Since some junctions are tight it's necessary to avoid understeering at slow speeds.
Anyway I'm quite sure that we'll see a lot of retired cars dug in ditches all around the route. To find the acceptable level of risk will be a huge challenge and previous experience with Belgian events will definitely help a lot.