I prefer the SM3DW style of game for 3D Mario (thought I don't like the timer and would prefer a little more "openness" in the levels...and it needed a better theme, like Galaxy). That said, your suggestion is interesting and I think it would be a big success if done properly. 3D Mario needs a few fresh elements. I would also add online multiplayer co-op and perhaps an awesome vs. mode where you grab power-ups and shoot fireballs at each other, etc in an open 3D arena. These things would add length to a title. Also, I think the only way an open-world Mario could work is if it was a hybrid. Perhaps an open world with 7 or 8 sub-divisions themed as grasslands or desert, etc, each with 3 or 4 linear courses to enter and complete. The sub-divisions themselves would be a huge open-world level with enemies, obstacles, missions, etc., but the goal would be to complete a few specific course runs that give you the classic feeling of completing a Mario level. I think it could be quite magical, really.
