Thoughts on a game design decision

In Forward: Escape the Fold, there will be a number of characters to unlock (I have around 8 at the moment) who will offer very different ways to play the game. The main objective will be for the player to progressively unlock these characters to explore these new gameplay mechanics. I have many ideas on how to unlock the characters but none seem perfect yet. So, by talking about them today, maybe you can help me choose :)

Idea 1: Purchasing the characters
My first idea was to spend gold found in-game (no real money obviously, the game will NOT have micro-transactions). It’s simple but there are two major inconveniences:

The player might find it hard to understand what he is purchasing using the gold coins as the description might not give that excitement.
The unlocking curve is tilted. The player could purchase a very expensive character (but would find the grind frustrating and exhausting) or if I were to block the purchases in order then using gold would simply be an illusion of choice.

Idea 2: Narrative integration
I thought about associating the unlocking of characters with the game’s story. It could be really cool because it would increase immersion and the game’s variety (the happiness of discovering a new card with some cool power). The biggest problem with this approach is its linearity, the player does not have a role to play and is ‘passive’ towards unlocking the characters.

Idea 3: Achievements
The current beta version uses an achievement system. Each character is associated with 3 objectives that need to be completed to unlock him. This allows the player to have mini-objectives and choose what characters he wants to unlock first by adapting his playing style. I developed this idea in the game and I’ve come across one major problem. Most objectives are unlocked by mistake by the player. In other words, he will unlock characters unknowingly simply by playing the game which is not what I intended!

Idea 4: Mission
For this reason, I think I will go towards a system that fuses the two last ideas. I am thinking of associating a “mission” to each character in order to unlock him. This mission will have an objective that will guide the player on how to play and what objects to use. This will reinforce the strategic aspect of the game whilst allowing the player to follow objectives as he sees fit. The complexity of these missions will naturally determine an interesting difficulty curve.

Conclusion
This is not a definitive choice and I’d really be interested in getting your feedback on this game design challenge (PS: I have never followed a training on the subject).

What do you think ? :)