Do you actually get to Slay the Spire? (And other questions on what's next)

[FearlessJDK]

I've beaten the main run a few times and attacked the heart.

Can you actually kill it? If so, how and to what end?

What other gameplay options other than the daily run or the custom modes can I unlock?

I love this game, but I want more to do. Having more than 3 floors would be fun.

[Vengyr]

To kill the heart, you need to collect 3 pieces of the key:

1. Dropped by the "burning" elite (guaranteed 1 per floor)
2. You can take from a non-boss chest instead of an artifact.
3. You can take at the rest site instead of other actions.

If you collect all 3 pieces, you unlock the act 4, which features the final boss.

Other than that, you can climb the ascension levels. They get progressively harder the more you win.

[Cranberry Shpee]

Not only can you get into the heart fight by collecting the shards, you can activate a custom "Endless" mode in the custom options.
And if that isn't to your liking, there's an Infinite Spire mod which provides a qualitatively different infinite experience.

[Khor]

I think just the Watcher gets to slay the Spire... the others just escape

[WolfWhiteFire]

First, once you beat the game with all three of the original characters, I don't believe watcher matters, but I am also going off memory with all of this so I could be wrong, you unlock special shards in your game. Each game, three will appear and the way to get them is constant between each game.

One, you can get by giving up the reward from a treasure chest for the shard, you can do this with any of the chests and the shard will disappear for that run once you have claimed it.

Two, you can get one at a campfire, choosing an option to get it and losing out on the opportunity to upgrade a card, rest, or do anything else at that campfire.

The third one is a special elite encounter, it will appear as an elite with flames around it on the map, and it will appear in each floor until you kill it, separate from the normal limit. It will be one of the regular elite enemies for that floor, except it gets one of many possible buffs that make it more difficult, I don't recall if you get the normal elite reward

.You need to collect all three in the current run, if you do, then at the end of the third floor when you beat the game, instead of going back to the title screen you will move on to a special fourth floor. If I recall correctly, it has a campfire, a shop, a special elite encounter, then the final boss. It isn't a traditional floor, there is only one way to go and all of those will be on the path, with the elite encounter right before the boss, and there is no normal encounters on the floor.

The elite encounter is difficult in itself, two powerful enemies, one on each side, when you take an action against one you get an extra 50% damage from the other due to it flanking you IIRC, I have only recently come back to this game so I haven't done that fight in a while, there is probably a guide on all this in the steam guides though, in any case, if you defeat those enemies, then you can fight the heart for real.

The heart is quite the difficult encounter, you will very rarely be able to beat it but once you beat the third floor it counts as a win whether you die on the fourth or not. It has a ton of hp, a limit on how much damage you can deal to it in one turn, each card you play deals one damage to you, with the boss buffing it's damage and the damage you take from each card every once in a while, it's attacks are powerful, and it is very difficult to beat, at least at my level of skill, which isn't much since I only ever got to ascended 3 or 4 before.

You might need a very specific sort of build to successfully kill it, but killing it is more for yourself and seeing the full potential of your build rather than having any actual impact or rewards.