Marksmen being too similar might be because of how auto-attack focused they are; There's only so many different ways you can design someone that is going to want to auto-attack as much as possible, especially if they're ranged.

Totally. There are ADCs who are less AA focused than others, and they tend to be distinctive. You can also do things with range (Cait vs Sivir). But the AA dependence is really what makes them frequently feel interchangeable or else drives some to just push others out since they occupy such similar niches.
At the same time, just de-emphasizing AAs risks making them feel an awful lot like mages if you don't add anything else.