An Obstacle is an anti-project. The effect applies before you buy it.
For instance:
Drought
$5 Obstacle
When you gain a card, discard your hand and lose all your $
Except for the combo-y exceptions, you can only gain one card per turn. Want to gain more than that? Better hit $5.
It might be interesting to design an obstacle that starts out bad but gets good later. For instance:
Baby Boom
$3 Obstacle
When you shuffle, gain a Copper. When you gain a Victory card, gain a cheaper card.
Can you handle the coppers long enough that your Provinces can come with Labs?
Another option is, rather than doing something mean to you, it does something nice for your opponents:
Police State
$10 Obstacle
At the start of your turn, each other player may reveal an Estate from their hand for +1 Coffers.
There might be other kinds of Obstacles that could be fun, but I'll leave that up to y'all.
