The interface to the feature is perhaps more important than the complexity of the implementation because it will affect many more people - only a few programmers will work on the compiler but tens of thousands of programmers will be writing code using it. I make no claims as to how complex this would be to implement, but it probably wouldn't stand out. The interesting thing is that this shouldn't necessitate any changes in the language of generic modules (no <>s and such). It merely parameterizes some types and constants in a module. As such, after a certain phase in compilation, the process is the same as for a non-generic module so perhaps it's a low-complexity change in the implementation, too (not just in the language spec).
You could, but the strings are not the language proper. There's, e.g., no relational expressions in your import strings (yet!), so there's no ambiguity in parsing it. I'd actually use parentheses anyway, since type parameters are still parameters (this could give the parameter list a "Pythonic" syntax which has been shown to work well already).
I don't fully understand your comment ("no relational expressions in your import strings") but parentheses are valid characters in filenames (thus URLs). I don't think Python is the best reference here.
