Stupid Euro Card Game Parody

Are you familiar with Fluxx or similar games? Fluxx's build-as-you-go is pretty easy to follow once you get the hang of it. Sounds, from my quick read-through here, that you'd only have one "RULE" card per round, so it's already a little easier than Fluxx where things, especially with a large group, can get flippin' ridiculous quickly. I might suggest you start there for a feel on games like this.

Another thing that you might look into, as it helped my often VERY lofty design goals come back down to reality, was Dvorak. Like Fluxx, you can use Dvorak as a "play 2, draw one" base that builds as more cards get played (or in the case of Dvorak, written then played).

I've done games like this, where cards are kind of dual-purpose and the best advice I can give you is take a very "underlying" approach.
First get the basics down: the cards that will be suited/ranked, played only for points, etc. Like if you wanted to use a standard hearts/spades/diamonds/clubs deck, I'd make/get those first. Then I'd write my basic rules. Fluxx starts with only "draw one, play one" after your initial hand, and then the cards determine what happens next. Dvorak starts with Action/Thing cards and I believe also starts with draw 1/play 1 (or 2).

Next ask yourself what are the mechanics you want to use in the game? Do you want to have people throwing a TON of cards around? Do you want alternate victory conditions? Do you want rules that will bend/break randomly? Brainstorm this effects/mechanics list out first, then take one idea and tinker it. A good idea is to limit yourself to three or four mechanics like hand size, victory conditions, point values, and drawing/discarding and then take them to their obvious, less obvious and just plain weird ends.

For example, one of my games that I've since kind of let rust is a modern fantasy game. I wanted to allow players a TON of latitude in how they could play cards-- item vs. ally vs. generic resource generator. Looking only at that concept-- multifunction cards-- I could come up with the following abilities:
Discarding my hand, forcing discard from opponent hands, assisting in battle, assisting in victory goals, making battle/victory harder for the opponent, giving extra draws, denying draws, etc. etc. etc.

That initial list allowed me to generate a ton of abilties that, while related to each other kinda-sorta, also felt fresh when recombined with others and cards just started popping into place.

I hope that helps. Here's the link to Dvorak (Fluxx can be found on Wikipedia/Board Game Geek).

http://www.dvorakgame.co.uk/index.php/Main_Page