Ahh I never even considered the cost of implementing artwork. Honestly if I were you, I'd start charging for the game with or without artwork - or perhaps make a patreon. I would certainly pay for a game like this.
A patreon is something I'm eternally considering on-and-off. Charging for MERCANOID now would leave a sour taste in my mouth, but not all future projects will be free, no. As my quality grows, I'll be more confident in charging for a game, allowing these older titles (and occasional free smaller projects) serve as my demos.
I don't know if this is realistic, but if someone were willing to create artwork and donate it to you (perhaps with the stipulation that the game said art is put into remains free, which by your indication is a reasonable expectation anyway for MERCANOID), would that make all the difference? I'm pretty sure I've seen a couple of primarily text-only games that included a small amount of fan-submitted art.
In this hypothetical, the best I can offer is 'maybe'. It would depend on the quality of the artwork, for one - this might sound harsh, but if the art doesn't sit right with me (or if it depicts the character(s) inaccurately), I'd have trouble including it.
For another, it'd require me to go back and update a game that I've moved on from fully by this point.
An official reference of Mio does exist, kindly drawn by a friend. It isn't fully finished yet, and I'm not sure on Itch.io's rules regarding the content of comments (even on a NSFW-marked project), so here's the clothed version.