updated 03:13 pm EDT, Mon March 10, 2014
Units may just be subtle revisions
Separate builds of iOS 7.1, available from Apple's website, appear to reveal the existence of two unreleased iPads. The files are for devices identified as "iPad4,3" (link) and "iPad4,6" (link) respectively. iPad4,1 and 4,2 are known to be variants of the iPad Air, while iPad4,4 and 4,5 are Retina iPad minis.
The unreleased tablets are therefore likely to be revisions of existing models, rather than major new products. Apple will occasionally do "stealth" updates of devices with more efficient components, the best-known example probably being the iPad 2. That product got a smaller version of the A5 processor, improving battery performance.