He's had a strange career. I don't think anyone would have given him a chance in hell in 2016. But he's been absolutely elite and one of the best players in the League since 2016-2017. I think Markus Naslund may be the best comparable right now. He really fell off after a dominant three year stretch. Marchand is continuing to roll along this season.
Assuming he has a five year stretch of being one of the very best in the game and nothing else worthy of consideration for the Hall (with seasons like 2016 being good but nothing special for players receiving HOF consideration), who would be the best comparable? John LecLair? He's not in yet. He would have at least as good a case as Theo Fleury then, no?
The fact that he's such a POS won't help him, but Ciccarelli didn't have a reputation for having great character and he got in.
let's say his last significant season is this one. that gives him three very high points placements and four very high PPG placements. This puts him in Naslund peak territory, offensively speaking.
Naslund has a more normal career cure though - aside from his four high-end seasons, his best PPG averages are 0.98, 0.83, 0.79, 0.73. Marchand's are 0.80, 0.79, 0.72, 0.65. So for him, it's more steep than Naslund, a guy who already isn't seen to have a lot of "meat on the bone" aside from his four best years.
But on the other hand, while Naslund was having these four excellent seasons (1.13 PPG cumulatively), he was scoring just 0.93 in the playoffs with a -6, earning a reputation as a choker (not all his fault). He wasn't known for anything at all, other than scoring points. Marchand, on the other hand, already has 47 more points in 56 more playoff games than Naslund ever had, and is still just 31. He's had four playoff runs (individually and team-wise) better than Naslund and his canucks ever put together. He's a quality defensive player and penalty killer, quite physical, and the most notorious agitator in the world. Remarkably, in his career he has 41 shorthanded points, despite being on the ice for only 107 PPGA. (for comparison's sake, Bergeron has 38 SHP on 243 PPGA). He's such a well-rounded player with a well-rounded resume that already checks a lot of boxes.
If he ends up having two more significant seasons after this one before falling off, the quick decline from his best seasons to the rest won't even be an issue because he'll have six excellent seasons, not to mention that in his first six seasons as a 2nd line-caliber player, he was still doing all the other stuff that makes him great (or at least makes him famous).
I think he's got a better chance than many other guys who have spent more time as a "star" caliber player - I think of guys like Eric Staal, The Sedins, Kovalchuk, Backstrom, Stamkos, Giroux, Tavares, Richards, Lecavalier... the way he's trending he may have a more attractive case than any of these guys, and some of those guys are going to make it.