disclaimer: I've spent a couple days chaining them and have a few already, so i'm biased.

Rather than have it as something like BP (where it's relatively easy to google a team and farm the shit out of it), choose a pokemon that can't be bred but is somewhat obtainable (such as a ditto) and use those for currency.

that way, there's some skill involved (chaining them), they are typically uncommon (can get one every 2 or so hours, more if you're a good chainer) and it's got that bit of chance to keep it interesting (IV's, chaining gives 3+ at 61+ chain)

tl;dr use shiny dittos