how about sticking to your theme of long distance, but combine it with kids. This would be something for 2009, though you should get some experience in 08 working with them...bit different to adults.
You could do a long distance walk-something a bit newish or whatever, but have pairs of kids along with you for a couple of days at a time. They would have to apply from around the US/world, and be selected. Age would need to be 14-18. Preferably they would already need to have some hiking skills and a base level of fitness. You could hold some training trips in 2008-possibly in combination with an organisation like outward bound, or NOLS, etc.
When it comes to the walk, plan it so that days are shorter than your average, but still a challenge for the teenagers. Its wholey dependent on terrain, but say 15-20 miles per day, and have them along for two, maybe three days.
Supply and train them with UL gear, etc.
You wouldn't have to have teenagers with you for all legs of the trip-and on these legs you could make the pace up if you wanted to. But yeah, pace shouldn't be an issue.
If you want international ideas...why not head to Australia? Doing the length of our Great Divide could be a serious challenge for you, considering the different climate (and climates on that trip), vegetation, gear requirements, water requirements, remoteness, etc. There would be alot of off-track work aswell. We also have some long distance trails in Australia like the Bibbillum track in SW Western Australia (~900km), the Heysen Trail here in SA (~1100km-the longest walking only trail in Aus), and the Alpine Track in Victoria and NSW (~600km).
We also have plenty of wilderness areas that have never been crossed...I won't list them, they are literally endless, and often involve deserts. When it comes to going bushwalking in Australia you tend not to think so much about trails but rather going off track. (unless you are a newbie).
Have fun deciding!