Resident Switch hacking scene member SciresM, known for his work on Atmosphere CFW, has made a curious discovery via datamining. A Nintendo Switch Pro model might actually be on the way, if you believe what he's uncovered from an update on the Switch. It should be noted that while interesting, this information does not confirm anything, and much is speculation on SciresM's part after having dug around some firmware files.
Existing under the codename of Aula, this hardware uses a Mariko SoC. It also would support both handheld and docked mode, and features "substantially" better cooling and battery life. While there's nothing concrete to fully determine, SciresM thinks that the revision would support 4K when docked, with an OLED tablet screen, as it features a Realtek chip branded as having "4K UHD multimedia SoC". If it's to be believed, then Nintendo has been working on a "Switch Pro" for over a year, which would line up with many reputable outlets repeatedly claiming that Nintendo is indeed working on a more powerful Nintendo Switch system.
Nintendo actually is working on a new model with upgraded display stuff and has been for ~1.5-2 years. I personally believe it's 4K given the signs I'm seeing in the firmware, but that's not confirmed yet and could be wrong.
Aula has firmware support some Realtek chip that advertises itself as a "4K UHD multimedia SoC", too, hence my belief it's 4K. That chip might be inside a new dock and not inside the tablet, though, there's no way to tell from the firmware code yet.
Aula explicitly supports handheld mode. It is not docked only; that's calcio, which seems like it's for internal only/never releasing since Calcio has no gamecard slot and no battery.
Checked my previous discussions with hexkyz, the upgraded tablet display is an OLED screen. Probably not higher resolution on tablet itself, so I guess the 4k realtek chip is more likely to be new dock than I thought it was.