At the very end of Fourth Stage when Takumi had the opportunity to sit in the passenger seat of God Arm's S2000. Ryousuke had the following monologue.. "D's battles will continue... burn the image of it into your eyes, for the sake of the more difficult battles that lie ahead. Completely absorb anything that's necessary. You'll find it useful someday."

The emphasis is on the last part --- that experience will help Takumi some day. One can't help but wonder if this is the author's way to lay the ground work for a secret technique that has yet to be seen thus far. If so, that'd be a great way to connect the series, making it a little more coherent.