Game Boy Classic release date
So far there's no word on a possible release date but we'd expect it to be next year at the earliest. The Game Boy came out a number of years before the N64 so it would make sense for Nintendo to continue in chronological order.
2019 would be a fitting time for release, marking the Game Boy's 30th anniversary, but we don't want to wait that long.
For now, the news is that is looks like Nintendo is indeed working on something Game Boy related. As Trademark_bot points out, Nintendo has filed a trademark for 'the first game boy'.
Similar trademarks have been filed, one of which for the SNES controller so it's a good sign.
The listing is a bit vague though as it mentions 'home video game machines', 'storage media', ' case for a smartphone', 'a computer peripheral device', 'a mouse pad, 'a computer', 'a notebook Type computer', 'tablet type computer' and more.
It also goes on to mention things like 'key ring', 'decorative item' and 'necklace' so Nintendo might be using it for merchandise.
It's the first mention, 'home video game machines', that is the most promising for a Game Boy Classic Mini but we'll have to wait and see.
Game Boy Classic specs
Nintendo could easily stick with the original form factor and dimensions of the original console and also pre-load the device with a number of games so you don't need cartridges. In fact, it's unlikely that existing cartridges would be accepted going by the other classic editions.
The biggest question for us would be the display and whether Nintendo would go for something matching the original or something more modern with a backlight, higher resolution and even colour.
Further questions include what games Nintendo will include – presumably Tetris is a given – and whether a link cable will work.