Good news for you :)
- Almost every first-party multiplayer Nintendo game on the Switch that I know of has offline local multiplayer. The only exception which comes to mind is Splatoon.
- The Switch has a cartridge slot, and leaks suggest the Switch 2 will too.
- And you can connect two (possibly more with a hub) Pro controllers with a true wired connection: https://en-americas-support.nintendo.com/app/answers/detail/...
Fingers crossed that the Switch 2 maintains this pattern.
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As well as local multiplayer, there's a feature to sync game updates without an internet connection. So even if people start off with different versions of a game and have no internet connection they can still play together.
I am a little disappointed they don't have anything like the DS's download play feature though.