Forge has stood out as one of my most played games, with even more play time than Skyrim, Minecraft, and a host of other excellent titles. In forge, I've almost exclusively done quests, and have a few ideas which could improve it.
Right now, quest mode is about collecting cards to build a better deck. Sometimes you may want to beat a specific opponent, but to do that you either need specific cards to counter-act there deck (
Energy Flux against Affinity, for example), or you need cards to improve an already existing deck. But it all comes back to one thing: collecting cards. The more ways quest mode has to do this, the more variance there is in how these cards come to you, the more fun quest mode will be, so here are a few ideas.
First, a new "Tournament" option might be fun, in addition to Duels and Quests. They could be Standard, Draft, Modern, Extended, Legacy, etc. I don't know all of the tournament formats, but there are a lot.
Draft mode: Drafting from a base of all cards just doesn't have the feel of drafting NPH/MBS/SOM, or SOM/SOM/SOM, where you have a consistent enough theme that poison or metalcraft are viable options, so you would want the drafts to be from a "block", as they are in the main game. Once you have built your deck, it goes to a tournament where you win extra packs based on your performance. If it's a multi-set draft, you would get to choose which set you want your prize packs from. Ideally (but unnecessarily), there would be a sideboard option, and the NPCs would play simulated games against each other so you fight against the best of them; however, that is difficult to implement (I think), so just having one randomly win would probably be good enough. They should cost just as much as the three individual booster packs, plus a little. So if it's Alpha/Alpha/Alpha, that would cost a leprechaun's sack O' gold. This could be fleshed out more, I'm sure, but I think it would add some nice flavor to Quest mode.
Standard/Extended/Legacy/Modern/etc: From your own card pool you would construct a deck for the tournament and then compete against a random set of decks which are legal for the tournament. The decks could be either pre-constructed or generated. Allowing sideboarding between games would be optional, but it would be awesome if it was an option. At the end of the tournament there would be some sort of prize based on both the cost and your performance.
I don't see how you could make constructed format tournaments scale with the players; however, they should be fine as just a high-tier goal. Maybe have them be unlocked after some certain quest accomplishments.
To really help with players wanting to build a specific deck, I think a singles shop should be created where the player can buy any card they want, but it's at a significantly higher price. It might be a good idea to have this shop open from a certain quest accomplishment (such as five hard quests completed).
Anyway, just a few ideas to add some length to quest mode. I feel like adding a draft mode could be done fairly easily, and would be a ton of fun; however, the other ideas may be a lot more difficult, or may not work, but perhaps they could be improved upon.