Just in case the idea hadn't occurred to you before now, you probably want to think about making those "cards" 3 x 5 cards, both for flexibility in having enough room to write on and the ease with which players can get more blank cards on which to write.
A game which requires you to pre-populate three by fives with data may not be ideal, but as long as the prep is relatively short or can be done on the fly by all the players at the table during Turn 0, it could come together fairly well.
The idea is that you can play the game with or without customization. Adding more cards can serve as a setup for multi-session play or make play with larger groups go better.
I'm thinking I might offer two versions; one with a 54-card "standard" set and one with a 70-card set. The print your own might be in 3x5 format, or I might just include a template for blank cards.
If I'm already stuck playing with a nonstandard deck of cards, meaning that I'm going to be required to print out my own stuff, whether it be on paper and then glued to card stock or directly on the card stock, it better be compatible with something that I can get my hands on easily and quickly so that the inevitable customizations that are going to come along at least fit in with the materials as designed.
At the very barest minimum, you need to include templates for 3 x 5 cards and for standard playing card sized stock.
Also, if a player action is something like "add a new location card to the deck and shuffle it in", and that does not leverage player creativity – you've just tripled your work as a designer for no real gain. If you want to design a board game, design a board game. But recognize the way that the tools get leveraged.
Of course, if you want to design a board game you will essentially be competing in one of the hardest fought markets currently available to entertainment designers, far more so than video games. With the rise of places like Game Crafters, board games and board game designers have access to high-quality custom pieces and printing for relatively low costs – making competition very, very tough.
One of the advantages of the RPG industry is that, truthfully, production value isn't that big a deal. People are perfectly happy with print and play work in the industry. Outside of the tiny niche in a tiny niche, however, there's not a lot of traction to be had.
Since brainstorming the original design, I've come to a couple conclusions.
Adding cards during play is probably too significant in a balance sense. You'd need to change several core mechanics in order to add them without causing issues. Likewise, it either requires us to slow down and go through a process (not ideal) or make these cards super-simple, which is not the goal of TPI.
Swapping out cards could be done as part of a storytelling conceit, but then you've got to question why you're playing such a game in the first place.
Formatting the cards is another issue. Poker cards are too small for most people to do by hand and have it work well. Fortunately, they're not a pain to do printing on (at least in terms of legible results). Originally my plan was to have a 52-card deck and be able to play with poker cards, but as I've watched the design evolve I don't think it'll work out smoothly.
My plan right now is to include the source files for the card templates so players can still add/remove cards from printings as they wish, and have an opportunity to print their own. Because none of the card creation will be during play, this gives people an opportunity to do this via computer, then print.
The biggest pain with 3x5 is that they don't print well. You get five cards to a page if you're going for efficiency, and since we're shaping up to have somewhere around a 60-70 card deck (this is an early estimate, and may not reflect the finished product), that would be a lot of printing. You can get 11 bridge-sized poker cards to a sheet of paper or 9 full-sized poker cards. I'm leaning a little toward the full-size poker cards, especially because they make everything just a little less crowded.
I'm probably going to wind up making templates for both down the road, but not immediately on release.