I'm really glad I found this thread! Sometimes when I get bored, I'll play around on gcmap.com and make my own Amazing Race, so I'm glad I have somewhere to post these now.
Here's one I came up with:
Starting Line: Malibu, California (Malibu Pier)
Leg 1: Brisbane, Australia
(pit stop at South Bank Parklands)
Leg 2: Jakarta, Indonesia
(pit stop at Taman Mini Indonesia Indah)
Leg 3: Labuan Bajo, Indonesia
(Komodo National Park, fast forward involving Komodo Dragons, pit stop at Pink Beach)
Leg 4: Kathmandu, Nepal + Pokhara, Nepal
(pit stop on Fewa Tal)
Leg 5: Mumbai, India
(pit stop at Durgadi Killa)
Leg 6: Durban, South Africa
(pit stop at Japanese Gardens)
Leg 7: Cape Town, South Africa
(cliff jumping roadblock, pit stop at Boulders Beach)
Leg 8: Ljubljana, Slovenia
(pit stop at Tivoli Park)
Leg 9: Saint Petersburg, Russia
(switchback to TAR 5 with the caviar challenge as a fast forward, pit stop at Winter Palace)
Leg 10: fly to Barcelona, Spain
, drive to Andorra la Vella, Andorra
, pit stop in Toulouse, France
(pit stop at Pont Neuf)
Leg 11: Montpellier, France
(pit stop at Place de Peyrou)
Leg 12: train to Paris, France
and fly to Detroit, Michigan
(task involving Motown and memory challenge involving old Ford models with license plates from the countries visited or with license plates that have airport codes, maybe a quick dip into Canada at beginning with the first clue reading "Welcome back to the United States, now go to Canada!")
Finish Line: Belle Isle, Detroit, Michigan
Leg 1: Elimination 11
Leg 2: Elimination 10
Leg 3: Fast Forward, Non-elimination
Leg 4: Double U-Turn, Speed Bump, Elimination 9
Leg 5: Elimination 8
Leg 6: Keep on racing
Leg 7: Elimination 7
Leg 8: Double U-Turn, Elimination 6
Leg 9: Fast forward, Elimination 5
Leg 10: Non-elimination
Leg 11: Speed Bump, Elimination 4
Leg 12: Double Roadblock, Finish
Miles: 34,059
Continents: 5
Countries: 11