A monotype I've been thinking about is Mono-ground for Platinum (can be done in DP, but trust me, it is better in Platinum)
Sinnoh has a large arrangement of Ground-types, including the following:
Turtwig/Grotle/Torterra- the one you should be choosing if you want to do a mono-ground run in the first place. Also means your rival's starter is weak to Ground, which is nice.
Geodude/Graveler/Golem: Admittedly, Golem isn't great in DPP, but he comes nice and early for a mono-ground run.
Onix/Steelix: Onix is horrible, but if you are lucky you can get the Metal Coat before you battle Gardenia by stealing it off a wild Bronzor. By doing that you have quite a powerhouse on your hands. If you don't want to go to all that trouble, just catch one on Iron Island.
Burmy/Wormadam Sandy Cloak: If you want to look like a fool, go ahead; I'm only listing this for the sake of including every Sinnoh ground-type.
Shellos/Gastrodon: The best of the Water/Ground types, and so the go-to option for a surfer in a mono-ground run
Barboach/Whiscash: Really just a worse version of Gastrodon, you can still use it though
Gible/Gabite/Garchomp: Let me think... Uber-rivalling stats? Check. Great ability for this playthrough? Check. Really you'd have o be crazy not to use this when it is at your disposal. Comes a tad late in DP though.
Wooper/Quagsire: See Whiscash, except Quagsire has slightly better stats.
Hippopotas/Hippowdon: Pretty bad in regular playthroughs, but in a mono- ground he provides awesome team support with Sandstorm, especially when you consider Garchomp and Gliscor have Sand Veil to abuse it with. A must-have for mono-ground.
Gligar/Gliscor: Not that great in regular playthroughs, but if has Sand Veil and you have a Hippowdon they can form a potent combo
Rhyhorn/Rhydon/Rhyperior: Again, not usually that great, but with Sandstorm patching up his Special Defense he can be quite bulky, especially with Solid Rock.
Swinub/Piloswine/Mamoswine: They come very late, but are quite powerful and can be useful against the Elite Four.
So there you have it; there are plenty of great options for Platinum mono-ground; this is probably one of the easier monotypes.