The bundled class and missions have quite a different flavour compared to base gloomhaven. The missions have a stronger puzzle solving emphasis rather than straight forward killing. The new class also requires a lot of forward thinking and planning rather than straight forward tanking/damage. This may or may not suit your taste. I suggest trying it on TTS before making sure its something you want.

So, in addition to being a sequel to the original game, FC improves it, too. Not just with a new class - though it is a *very* nice class - but with some rules clarifications and modifications. If, like me, you've had trouble remembering how (and when) monsters focus during their turn, the rulebook includes a clear step-by-step reference on the back. If you've wondered how some classes abilities to just poof across the map might be weirdly power-balanced, the new teleportation rules fix them.
The fully new content - the Diviner seems like a dream class for someone like me who can have an attack deck of 24 cards with only positive modifiers and one miss, but still draw that miss one time in four. (No, really. I'm cursed). Diviners break the fourth wall and let you look at and fiddle with draw decks. Very nice.

EDIT: Having played the campaign through, I'll admit, I'm less impressed now. The Diviner is a necessary character, and we're only two players, so that meant we only had one attacker. Because the Diviner, despite a nice draw-deck, does not do a lot of attacking. Or healing. They're very much a support class, from what we've seen, which would be fine if we had a choice of them in missions where you need to kill some big guys real fast.
Additionally, the quests wander a *lot*. Much like the main game, there's a single plot through-line, then a lot of side quests. However, these side-quest can be multi-quest plots themselves. And it gets kind of hard to keep track because you can't just go back to the last map to re-read the plot - you have to follow through three or four (or more) choose-your-own-adventure-style page flips. Which makes it real annoying to find a single detail you've forgotten, especially if you can't remember which map it was in.
And finally, the page-flipping 'portal' gimmick is amusing the first time. Ooh, we don't know where the portal leads, gotta go follow a string of tidbits across a sea of pages. But in reality, for a whole campaign, it's tedious and annoying. You interrupt the game two to three times a session on average to create new map sections, sometimes requiring enemies that weren't in the set-up, so you have to fetch more pieces and so on. By about halfway through, we were sick to death of this mechanic. Having several bookmarks and having to flip back and forth across pages repeatedly to recheck rules, find details, etc. got to the point where we just wanted it to end. We would stop playing for weeks at a time due to frustration with the game. Eventually, we just cheated, figuring out exactly which quests would open the pathway to the finale so we could skip half a dozen quests in between and just get to the end. I can't really recommend this one now, having played it. The Diviner isn't terribly fun until maybe level 8 or 9, and they tend to act more like baggage than help in some quests if you don't have a big party (seriously, its like the class and quest designers didn't communicate at all - in some places the Diviner has no way to help with the actual goal. And yes, other times, they're the only one who can interact with the necessary components, but then the other players just become canon fodder, fighting monsters while the Diviner does everything. Neither of these was particularly fun for us.)