They could, but they need to decide wether they want to.
Than they would have to ramp up the game production, making parallel releases of the casual games with the hardcore ones (no long gaps in releases).
This would result in weaker sales of the games in the long term, but would maintain momentum of Wii.
Nintendo releases are atleast 90% hit and 10% miss, so their almost every release would be percieved as a quality game.
They have lots of established IPs, so that ain't the problem also. Even if they decide to bring some old IP's from the dead, it would create a decent hype, and therefore momentum.
If necessary, nintendo could buy another dev team or two (they certainly have the cash). Judging by previous examples, those would be young and unestablished teams, but with lots of talent, so they wouldn't be expensive to buy, and Nintendo has vast experience towards directing talent into quality products.
But I highly doubt it N wants that situation, because they like to maximise their profit, therefore with lots of releases in a small timeframe games would eat eachothers sales.
Also, by adding more dev teams, N would grow larger and therefore making the company more prone to sudden market and industry shifts.
.
