I don't think so, or at least I hope they don't do that. I can believe Marco just never let himself have feelings for her the way Star tried to do with him, but I don't think the Blood Moon Ball started that.
Feels too "lol friendzone" for a kids' show to me and I guess could be taken as sending a bad message almost: if someone doesn't like you, it's YOUR fault (and just not them sharing your feelings at this time). Also makes Marco seem thick as hell to take "we don't need to save each other all the time" as "EW FUCK OFF I DON'T LIKE YOU THAT WAY" and keep feeling that way even when Star's strangely weird about his relationship with Jackie, especially when the episode just before this, Sleep Spells, was about the same subject of being "a hero." And finally I just don't see anything that indicates Marco was crushing on Star at any point prior to that and then suddenly stopped because she told him to.
Also one more thing I guess is that it seems weird to refer all the way back to that one scene in that one episode and say "yes this totally changed my feelings completely and has influenced the way I act for multiple seasons since then." Rather than him simply not feeling the same. Like it's all some big dumb wacky misunderstanding that went on for literal months.
Starco will definitely happen; I don't even ship it myself. I just think Marco being revealed to have a long-time crush on Star that he hid as well is the wrong way to go about it. I think it'd feel more natural for him to just start developing feelings for her too, after or before the inevitable breakup with Jackie.
I don't think it would have been better for the show if Starco happened earlier because do we really need dumb couple bullshit for over three seasons? At least right now things are developing and changing; if it happened right now it'd just be "aren't we cute" over and over, probably with forced one-episode drama too.