The next launch is critical. A launch determines the initial customer base, which influences which games sell and which ones don't. Which influences which games the system gets and which ones it doesn't.
I don't think Nintendo can do anything about their customer base ths generation, even getting away from the kiddie image won't suddently bring in enough customers that want violent games to give those games decent sales numbers. And therefore those games won't be supplied by most third parties.
Nintendo should make sure they grab one or two of the most important markets at launch (e.g. exclusive sports games or RPGs or something) to get customers who buy those kinds of games and offer third partes a market to release those games in. If the Revolution was seen as the big RPG system, I'm sure Square Enix and others would follow.
The problem is, the only market Nintendo aims for is the Nintendo market. As long as they keep reusing their franchises the games will appeal to the same people, even if they don't change the gameplay they need to create additional franchises that aren't too similar to the current ones. Using less known franchises would help, too (e.g. Fire Emblem), but they shouldn't over exploit the franchises.
Nintendo should analyze which segments of the market are profitable and try to take them early on. The more of that market they capture with the first strike, the more third parties will help them conquer it.
I think sports and RPGs are the two main markets worth aiming for, RPGs sell big in Japan and sports lead the charts in the west. Having those two will give you the majority and might reel in a few smaller markets as well.
Since Microsoft cancelled their sports lineup, Sony has none, either (at least to my knowledge) and both don't make the RPGs themselves, Nintendo could get an advantage by servicing those markets with maybe two games (well, two good games or one killer app) each on launch (or shortly thereafter). Microsft got the FPS market with Halo alone, Nintendo should be able t own the RPG market with two games early on (hm, maybe a new franchise and the full Pokemon RPG people are suggesting for so long now)