Here is a proposal:

https://gist.github.com/vlopezj/1de925406d10eba30f6fd849ed174917

TL;DR: Implicits would appear to the right of the part of the mixfix operator they fall under. In particular, this means that sometimes the arguments to a mixfix operator will appear before the implicit argument they depend on.

UPDATE: (per @UlfNorell) Implementing this proposal is complicated by the fact that there are more places where an implicit argument can appear in a type signature than positions in an infix application where an implicit argument can appear (i.e. just after a token). Which position the implicit corresponds to can only be determined once the type of the infix operator is known.