WayForward is rewarding its true fans who buy both Mighty Flip Champs and the upcoming Shantae: Risky's Revenge, with an unspecified bonus in the latter. If you have Mighty Flip Champs installed on your DSi, you'll find something special in the "import room" in Shantae's Scuttle Town.
The exact identity of the bonus will depend on your progress in Flip Champs, but, as WayForward's email announcement notes, "Whether you bought it, beat it, or rank among the best, a bonus awaits every MFC player." If you don't want to feel left out, you can still buy Mighty Flip Champs now and work on it until Shantae's October 4 release. For that matter, there's nothing stopping you from working on Mighty Flip Champs after Shantae's release.