Even in Sailor Moon, there are girls who wear cloths and behave in ways that are generally associated with masculinity, "Magical Girl/Boy" doesn't just mean skirts and pink, despite the popular narrative.
Originally, "Magical Girl" genre was created partly as a toy seller like many of the newer and popularised genres in the 80s to 90s (I'm looking at you, Mecha genre), but the real thematic reason for the genre's creation was ultimately meant to discuss gender roles and identity. Through the use of Magical Girls, girls are led to explore aspects of comradeship, individuality and personal freedom, taking initiative and actions, and even the use (and correct use) of violence and otherwise things are traditionally viewed as not suitable for the role of ladies where girls were expected to be soft-spoken and well, you know the deal.
Similarly, with the popularisation of Magical Girl genre, a reverse genre called "Magical Boy" was created for the exact same general purpose just in a different direction, where it leads boys to explore aspects often associated with femininity that are traditionally deemed as inappropriate or to quote your everday-joe, "gay".
So no, personally I don't believe any sensible artist and designer would wind up just giving Ezreal a skirt in the supposedly upcoming Star Guardian skin. It will be more "fluffy", but very unlikely to tread into the aspect of skirts, considering Star Guardian skin-line wasn't exactly made for comedic purposes and they seem to take it quite seriously as its own thing, which in that case I'd also believe they'd handle the subject in a smarter way than simply putting Ez in a skirt.
/2cent