I never said he wasn't his own person, just that he doesn't have much distinction as a character. He's had a single game [technically three, including DS-exclusive Luigi's Mansion 2 and a Dr. Luigi game, with a fourth game [Luigi's Mansion 3] coming out soon; however, people typically only think of the first Luigi's Mansion], and a meme, and that's really it; Even the least favored Robin has had multiple significant character arcs, and several of them (Notably Dick and Tim) have had their own high-profile series, with Dick even having gone so far as take up the mantle of The Batman multiple times. I enjoy playing as Luigi in most Mario games- as a rule he's quicker and trickier, which suits me far better than Mario's turtle-y brawler approach; however, he's still just a side-character who is given the occasional nod, rather than someone with a clear and firmly established separate identity.

My point was less to make any implications toward Luigi's side of things, and more to make a point that most Robins have gone well beyond being throwaway sidekicks. Damian is the newest and most shallow, but he's nevertheless had significant impact on Bruce and Dick. Stephanie is basically matched to Princess Daisy- spunky, determined, surprisingly gritty, and utterly ignored whenever an arc involving her is over. So at best, Luigi'd match to one of those two, and neither of them are typically the Robins people think of when they mention "Robin".
Ace fails by virtue of being too obscure, compared to Luigi, but he's got the same goofy awkwardness and shoved-in feeling, so he probably matches best on character implementation. Which really does say a lot about how hyper-focused Nintendo's approach to their franchises is, if their secondary character in a franchise can be so easily matched to a.. I dunno, quinary? character in another. It's not like the Mario franchise is lacking in established side-characters compared to Batman, either, despite Batman's surplus.

Anyway, I'd totally appreciate games based around more side characters in general, be they games based on Nightwing, Teen Titans, or Luigi. Heck, I'd be all over a Princess Daisy game [where presumably she'll approach situations through methods other than crying her way through them, as per the methodology of the Princess Peach game].