9/12/2016 - 'Modular' is the wrong word because it already means something in Magic. Ignore that. (Although these do play well with modular creatures.)
The concept applies though. Well, okay, the normal English version of the concept does; the Magic meaning is also already taken and means something else (although this is a somewhat modular mechanic).
I'm not helping. It's a weird mechanic, so I'll just show you. Then we'll talk.
Modular Cockatrice gains an ability when you put a +1/+1 counter on it. What makes it modular—and not simply Baloth Pup, Lightwalker, or Dead Reveler—is that you get to choose which ability it gets. What makes it weird, is that you mark that choice based on where you place the counter on the card. And if you get a second counter, you can have both abilities. If you want them.
I chose Cockatrice as my reference, because it's a card that has a lot to gain from this mechanic: It has two useful abilities that aren't quite as useful together. Modular Cockatrice lets you choose which one you want when you Battlegrowth it, and whether you want both abilities when you do that again. (And I replaced Cockatrice's proto-deathtouch with lure because I misremembered. The point stands either way.)
This immediately presents logistical issues: You'll have to be careful moving your card about or your counters will slide out of place; and you're forced to use counters that fit. Fortunately, the traditional coins, glass beads, small dice, and rips of card/paper all do, but that remains something Magic has never codified before.
|Forgive my hasty photochop: The circle in the illustration is hiding in the top-left.|
On maybe one rare, we could create a push-your-luck thing, where the first counter is great, but the second counter is terrible, making you reconsider cards that add 2 or 3 counters at once. Except, can't you just put counters elsewhere on the card? Are you limited to how many you can fit? You aren't normally, and players usually use dice when there are more than a couple. So the downside approach is useless. What else?
Modular Nighthawk is neat, sure, but how many cards can we make like this, and will that ever be worth the graphical treatment, communicating how the mechanic works to players, and the physical logistics it imposes?