I think 12 liters is sufficient. You get about 300km out of a regular tank, 400km if you stretch. add 12 liters you can almost double that. When will most of us ever need more? Except for the most remote locations a reach of 600km is totally fine. For most of the real world users anyway, most of them probably can do entirely without the tank. Less weight though - is always a good thing, right? Also it would be something to differentiate it from the Touratech option (if the price won't be that factor anyway).
And yeah - we need pictures!
Edit: Of course on could always argue that you don't have to entirely fill up the bigger tank... I'd still think I prefer a smaller, more slim and lightweight solution. It'd also be great if your design, other than the Touratech one, would take into consideration the air filter in a sense that you can still easily get to it.