I've developed a game concept around a theme first, and a few interesting card mechanics. Then I moved on to integrating these elements into a larger structure to "house" the rest of the game play -- namely the passage of time and a way to mark progress through a region.
Basically, the game has a variable endpoint based on the passage of time. Players maneuver through a series of cities, the more cities you visit, the more time passes and the greater the chance of the game ending.
However, as I was spitballing ideas, I reached a fork in my brainstorming -- implement all of this via cards or develop a game board to track these things?
There are pros and cons to each... Thematically, both work well. Mechanically, both work fine. The real question is, which *feels* better? I've got no clue!
Card game considerations: Cheaper to produce. Smaller box. No need for extra bits. Endgame can be easily adjusted by changing where certain "trigger" cards go in the deck. Requires substantially more artwork for all the different cards. Requires slightly more set up time (to split up and populate different decks). Requires score be tracked on paper. No impact on length of game.
Boardgame considerations: Easier management of progress/location as its visually tied to the board. Scoring track eliminates need for pen/paper. Higher production cost. Larger box. Less artwork, as art can be tied to gameboard and some cards turned into tiles. Quicker setup time. No impact on length of gameplay. Provides slightly more tactical "feel" to the game, even though decisions are the same, due to the visual representation of goals. Easier to integrate passage of time, as it can be tied to board elements instead of cards shuffled into the play deck.
I'm reluctant to develop a prototype of each version, due to the time commitment, but wonder if that might be the way to go. When all else is (roughly) equal in terms of theme, gameplay experience, etc... do you prefer a card-based game or a more traditional board-based game?
One question to consider is the scope of the finished game. A card game has the feel of a smaller dedication of time to play and a "lighter" feel overall. A board game will lend a more epic and involved feel to your game.
That doesn't mean that every game with a board is either epic or involved, but if you want your game to be more of a 1.5+ hour and detailed game, then you may want to go the boardgame route.
Something else to consider would be a slight hybrid between the two . If the board is mainly for tracking game states, then you could make your game a card game but with a simple card instead of a full board (think Modern Art or Camelot Legends). These are still primarily card games, but ones with expanded features. The production cost of a game such as this would still be closer to the cost of a card game (either game I mentioned can be found for less than $25).
Best of luck!