It's possible that Nintendo had planned the Switch's overall concept as soon as 2006 with the Nintendo DS. However, the company seemingly scrapped a feature that would've allowed players to easily connect their DS Lites to their TVs.
Many gamers consider the Switch to be what Nintendo wanted the Wii U to be back in 2012, but because of technological constraints such as battery life and the limited connection between the GamePad and the base, the Wii U never lived up to previous console reputations. But the Wii U was unique in that it allowed players to use the GamePad either connected to the TV or as a handheld, though it doesn't act as a true handheld in the way the Switch does. Wii Us aren't meant to be played on the go because the GamePad can't separate too far from the console. But it looks like this idea may have been incited as early as 2006.
A hidden feature was discovered by the Lost Nintendo History team late last year, though it's just recently gotten attention on Twitter. Essentially, at some point, Nintendo planned to add a video output feature that would send a signal of the top DS Lite display screen to the player's TV while using the DS Lite as a controller, not unlike what the Wii U and Switch would later go on to do.
Lost Nintendo History, which is geared towards uncovering and restoring the secrets of Nintendo's software and hardware, discovered how to mod the DS Lite's software so players can implement this feature on their own. According to the site, all DS Lite consoles were built with this ability in the hardware, but the original DS and DSi consoles were not and cannot be modded to implement this feature.
For those curious enough to try this out on their own, Lost Nintendo History provides a step-by-step guide on modding the DS Lite's software, but this requires hacking into the DS Lite as well as adding a few extra hardware components to make the feature possible. It involves downloading custom firmware, homebrews, and installing the Twilight Menu onto the DS Lite, which is basically an upgraded, fan-made menu replacement for DS and 3DS consoles.
Some theorize that the reason this DS Lite feature may have been scrapped is due to the DS's resolution. It features pixelated graphics that look fine on its own screen, but wouldn't look very good on a TV several times larger, much like playing DS games on the Wii U or even old Game Boy games on the Switch, available through the eShop.
Source: Lost Nintendo History