If we're gathering pipe dreams, my contributions would be SSiInuktitut, SSiRomansh, SSiMaltese, and SSiNiuean!
Inuktitut and Romansh have the disadvantage that, like Welsh, they don't have a standard dialect, so you'd have to pick one (two main contenders for Romansh - Sursilvan and Vallader -, and for Inuktitut I suppose South Baffin is the one that comes to mind given where the capital and biggest settlement is).
As for Lojban, I'd be really curious how well that would work out! The grammar is something quite different from pretty much any natural language, and I've heard it claimed that learning the entire language is impossible for humans. (Though subsets that map more or less closely to how other languages work are easier.)
I wonder how much the twins brought up learning Lojban ended up being able to use productively.