I expect a combination of the above. More third party support is critical, but I also think Nintendo should be moving to expand their first party studios--either through acquiring existing studios they already work well with or by setting up new teams. I don't expect to see massive innovation in control interface, but perhaps a form of 'soft' innovation like 3DS' social connectivity/wireless features, with a more obvious pull (like 3DS' 3D screen) to grab headlines and wow audiences initially. I definitely think Nintendo should move to have a machine that has graphical capabilities on par with or above the PS3, to help bring in more 3rd party support, as well as an improved online structure and serious integration and connectivity with 3DS. Considering the big deal they made about GBA/GC connectivity several years ago, Nintendo have done very little with what would have been easier (wireless) connectivity between DS/Wii.
EDIT: I'd also like to seen an expansion of Virtual Console as a cross platform service--you could have one account for both your 3DS and N6, and play your VC games on both these systems. Perhaps start playing something like Secret of Mana on your N6, then transfer it onto 3DS if you're away from home at the weekend and wish to continue playing. Obviously being able to transfer over your existing WiiWare/VC purchases is a must--if Nintendo didn't include this in their next system I would be seriously put off.
