If Red Dead Online emerges from BETA and becomes like an "official" game next year, then it'd be that. But, similar to Fallout 76, even though both games are clearly unfinished, they're squarely 2018 releases.
To come up with a flop it has to be a really high profile game, so that's why people are throwing out AAA games. If some lesser known game that nobody cares about flops, then it's not really a flop. It has to be a game with a big budget and a lot of promotion.
I think it's probably going to be Anthem or The Division 2. A lot of stallwarts won't allot first party games to be considered a flop, and I think ultimately Sony's studios delivers. But, third party games released by major companies are prime for flop territory. Anthem seems to be the one that's primed the best because it's probably going to take a games-as-a-service approach to content, launching kinda thin, disappointing at launch, with technical issues, and then slowly add more... Akin to Destiny. When Destiny launched I would have called it a major flop, but with subsequent content releases the community ended up coming around on it... but back in 2014, Destiny felt like a major flop to me. Today I don't think many people would say that Destiny was a flop, but in Sept, Oct, Nov 2014 when it came out, it certainly felt like it came way short of expectations.
So I'll say that Anthem most fits the 'Destiny' like blueprint.
For sure, Anthem will do better than Far Cry 6 (or w/e it is), Rage 2, etc., but there's not that much hype around those games, at least compared to Anthem.