I'll predict that Paper Mario 3DS is somewhere in the pipeline. The prior games were released in 2001, 2004 and 2007 respectively - going by this three year interval schedule that the series has adhered to, I'd say we are due a Paper Mario this year or early next.
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Not much of a basis for a prediction. A better argument for a new Paper Mario would be the fact that this franchise has previously mixed 2D (which has a strong pedigree on handhelds) and 3D (which is evidently the 3DS's speciality) together. I'm not sure the New Super Mario Bros moniker would be used for experimental 3D gameplay, but it would be perfect for Paper Mario, known for being quite experimental.