I could be wrong but I have suspected that one of the main reasons so many Wii and Nintendo DS games seem to have such long legs when it comes to game sales is because people are constantly introduced to these games in a multiplayer setting. For games like Mario Party, Mario and Sonic at the Olympic games, and even the ever so hated Carnivale games people were exposed to these games in a social setting and (because they enjoyed themself) they bought the game to play by themself or with other friends; and the same (basic) thing has happened with dozens of other games including Mario Kart and Guitar Hero so far this generation.

Now, the problem with Online games is that they require people to own the game in order to play it in a multiplayer setting and (therefore) it prevents people from being exposed to the game enough to become interested in buying it. With this in mind, I think that Nintendo should take a page from the Nintendo DS' book and create a framework for "Single Cart" play on their home console ... and here is how I would do it:

Nintendo should allow (and encourage) developers to put barebones clients on the WiiWare service for free, and if someone wants to play a game against their freind (or with a group of friends) they should be able to download the necessary content files from their friend in order to play with them.

Now developers would (probably) limit the "Single Game" based multiplayer to a limited set of small maps, but even this would enable more people to be exposed to their games and they would (probably) see much higher game sales.

All is needed is to learn from Mario Kart Wii that is still so popular and played online

Or just add local multiplayer and let people combine local play with online like in Halo 3

Hmm, this will be very limited by the fact that the wii has 0.5 GB of internal storage, and the disc drive can hold up to 9.4 GB (dual layer)

Good idea for The Conduit, i think. Load in a single map, have some fun with a quick match, etc.

Umm... plenty of Wii games do this.  Mario Kart Wii, SSBB, Mario Strikers Charged - just not the shooters for some reason.

@OP: I had a similar idea, but it's limited by a few factors:

1. Nintendo would likely have some kind of Friend Code-related interference here, and who wants to add Friend Codes just to see if they like a game?

2. If the download was limited to 40MB like other downloads on the system, you likely wouldn't be able to get much of the game, as since you have no disc in the drive the entire game package would need to be included in the download.

Good idea, though, in theory.

I could have sworn Nintendo stated this exact thing would be possible back probably in 2005