That "mostly" in "mostly unchanged" is a problem. Have should probably be either significantly radically different from Go, so that when I use it I'm not confused, or it should be as similar to Go as possible except that it adds some features, so that it seems like a nice add on.

Rewriting syntax that ends up really just being a gloss on the existing syntax (i.e., not really allowing much new expressiveness) is just a hurdle to the language; it's a hurdle to your implementation, a hurdle to anyone picking it up and using it, and a long-term burden of trying to explain the slight differences to everybody. See, for instance, the Python 2 vs. Python 3 pain.

Since it seems to me the major feature of Have is giving Go generics, I suggest comparing with some other things already out there: https://github.com/sasha-s/go-inline , and I also reference the projects mentioned in the FAQ. I am not saying these things solve the problem, but you may want to compare notes and steal (in the "good artists borrow, great artists steal" sense) the good things if you can. You might also find it helpful to contact the authors or users of those projects and ask about pain points.