I remember reading something about Nintendo trying to buy Bandai many years ago, but that was apparently blocked by some government intervention. Could be wrong. But if i remember correctly, this means you can't go around buying companies just because you have the money. At least not in Japan.
But buying Bandai-Namco would probably be their best bet. Then they not only acquire meaningful ip's but also a stronger foothold into the anime and toy industry. They'll have an easier time diversifying, in which they seem to have a need to. But then again, maybe it's not a good idea.