By diversifying their own 1st/2nd party output.
If say .... Halo was a Nintendo in-house IP, I'd venture a bet that things like COD/Battlefield would probably sell better on their platforms.
Nintendo is the only big publisher that doesn't have a AAA blockbuster IP aimed specifically at older players. Konami has Metal Gear, Activision had Call of Duty, Ubi Soft has Assassin's Creed, Square-Enix has Final Fantasy/Tomb Raider, EA has Madden/Battlefield/FIFA, Take 2 has Grand Theft Auto, etc.
The closest thing Nintendo has is Metroid, and Metroid simply isn't a big seller (really has not been for 20 years now).
The reason third parties still keep bringing out things like LEGO, Sonic, Just Dance, Skylanders is because the overwhelming majority of Nintendo's own content is cartoony/mascot based titles or party games.
