this concept of a support marksman would only ever be possible to balance if it was an ap marksman who could itemize either into utility and be a support, or into damage and be a carry.
the problem with the existence of a support marksman would be that the idea itself is ridiculously broken. all marksmen scale into lategame with strong auto attack damage and attack multipliers to the point, where theyre stronger than almost everybody else when it comes to objective control and champion damage.
if you would add the utility and base values of a support onto that, you would end up with a champ with an incredibly strong early game, an incredbly strong lategame, and amazing utility.
you would put the power of two very distinct roles into one champion.
knowing riots boner for marksmen, this is probably going to happen at some point. i mean they made
into the abomination he is, so it wouldnt surprise me anymore.