I would say definitely not. They could do much, much more with the rally stages and actually maximise what the rally cars they have are supposed to be used for. I thought the use of stages was good in GT5 but the execution could be greatly improved, such as using real-life, or otherwise decent permanent stages, or at least making the randomly generated stages more varied and interesting.
I think they should also focus on the rallying gameplay - in my opinion the rallies should have 11 rivals, they only show the ghost cars of the rival who set the fastest time in a given stage. Waiting 40 seconds at the start of a stage is just aggravating, and no rally releases cars at 10 second intervals anyway. This, coupled with stages a mile wide just so that the player isn't hopelessly stalled behind slow AI is neither realistic or particularly enjoyable. By showing the ghost cars of the fastest cars in a given stage, then there is the chance of seeing different rivals in different stages, rather than the same three. That is, if ghost cars are even needed. Colin McRae rally didn't need ghost cars to make rallies engaging.
So in my opinion, they shouldn't get ambitious with other offroad series without properly fixing the gameplay of the current rallying system. If they don't, it will feel very second-rate when GT6 is released.