Big impact would be S-E or T2. Square would give them a stranglehold on the Japanese market, encouraging all Japanese 3rd parties to support them. They also have a very significant back catalogue, and several western devs to boot. T2 would provide that massive and influential western IP that would move systems.
If they didn't go the big name route, they should buy or contract a series of smaller devs to make core games. Big Red Button, Rocksteady, Crytek UK, Edge of Reality, Slightly Mad Studios, Mistwalker, Platinum Games, Comcept, etc. could all be purchased for less than a big publisher and could make new IP's in genres like shooters or racers or adventure titles, and could be given their own unique publishing brand like Nintendo Worldwide Studios. Studios like that could give Nintendo fresh game concepts and massively increased game production capacity, making them virtually independent of third parties for content.