Splatoon is essentially based on multiplayer sessions. Unfortunately, it seems Nintendo do not intend to provide an offline mode for Turf War Battles, which is the most interesting mode of Splatoon. Meaning that:

• You cannot play Turf War Battles with your friends unless each player plays on his/her Wii U.
• If you do not have an internet connection (e.g.: power cut), you won't be able to pratice on Turf War Battles maps.
• In 5-10-15 years, Nintendo will surely disable online service for the Wii U. Splatoon will simply die, and no one will EVER be able to play Turf War Battles.

We are aware about the issues which Nintendo is facing for Splatoon:

1. Lack of power to manage several players.
2. There is only 1 Wii U Gamepad available.

Nintendo needs to make small changes in order to make offline mode possible.

 

Lack of power to manage several players

As shown by Nintendo, the Wii U is able to manage at least 2 players. One is playing in HD resolution (probably 720p) while the other is playing on the Wii U Gamepad in SD resolution (probably 480p). In my opinion, I think it would be possible to split the screen for 4 players (even by reducing the frame rate, as it is done in Mario Kart 8).

Nintendo could let us experience the game in 2v2 and/or add bots to fill the teams to reach 4v4. The single player mode already implements an AI for them, it shouldn't be that difficult to adapt it for Turf War Battles.

In addition, if Nintendo really wants to make everyone happy, it could be possible to use 2 Nintendo Wii U and 2 screens to organize 4v4 matches. This is something that has already be done in the past (e.g.: Halo on Xbox).

 

Only 1 Wii U Gamepad available

Unlike the Wii U Gamepad, the Wii U Pro controller do not have accelerometer/gyroscope. However, the Wii U Pro controller could still replace it: it is possible to aim using only the second stick, as shown by the 'cheap' offline multiplayer mode presented.

The Wii U Gamepad has also another screen which is used to show a map, and allow the player to 'jump' to reach another spot. The map could be displayed on the screen. In Splatoon, you cannot jump wherever you want (there are only a few possible spots: your allies or a platform set by a player of your team). A button/special manipulation could be used on the Wii U Pro Controller to enlarge the map and select the location to reach.

By doing this, everyone could use a Wii U Pro controller.