I also want some kind of syntactic sugar for H. Lists.

The absence of syntactic sugar makes heterogeneous list to look scary. People in haskell is accostumed to syntactic sugaring, so people think of not sugared expressions as second class. 

IŽ don't know the details, but it seems that tuples are in the language to cover the heterogeneous flexibility thing that homogeneous lists may not provide, but they introduce its own inflexibilities; These repeating instances for two, three, four and so on tuples makes them artificial and repetitive. In its comparison, a complex but unique H. List instance look elegant. specially if it is sugarized.

An alternative to sugarize H.Lists preserving tuples could be to use {} to sugarize H. lists as 

   {x,y,z}

and desugarize it into:

x :*: y :*: z :*: {}

Just like [] means empty list, {} would mean HNil, the empty heterogeneous list.

But this alternative , if implemented, would soon render tuples  obsolete. These (,,) (,,,) constructors may create marginally faster and compact structures, but they are much less manageable.