In terms of the spelling for a type sequence, if using a prefix * for the splat operation, might it not make sense to use * on the other side, too? So instead of T... use T*. Then you could spell that potentially undesirable matrix example as func matrix<T*>(_ xss: (T*)...) if I'm understanding it correctly.
I don't know if that's really better than the dots or not, but... maybe?