Instead of energy cards, we use other Pokemon. In place of the usual rule that allows you to attach one energy card to an in-play Pokemon per turn, you can sacrifice (ahem, "tribute") a Pokemon from your hand or the Bench and use it like an energy card of the same type (so a fire Pokemon behaves like a fire energy card, for example).

Another modification we made is to the summoning/evolution rules. AFAIK, in the official rules, the only (normal) way to put an evolved Pokemon into play is to play its lower forms first, laying each new one atop the previous canonical form. In our version, any Stage 1 Pokemon can be evolved from any Basic Pokemon, and the same for a Stage 2 from a Stage 1. When "summoned" this way, the newly-played Pokemon card retains all the energy (but also all the damage) attached to the Pokemon it replaces. Stage 1 and Stage 2 Pokemon can also be summoned directly to the Bench (that is, as opposed to being played upon an existing in-play card) by tributing the same number of 'mons as the Stage number (so, 1 or 2 respectively) from the hand or Bench.

That last part is crucial; you can't tribute your Active Pokemon, because then you could cheaply just replace a heavily-damaged 'mon with a fresh one. But you can do it in a more roundabout way by first paying its retreat cost to return it to the Bench and then using it as a tribute, either to summon another 'mon or as energy.

Lastly, we allow retreat cost payments to be supplemented by tributes from the hand or Bench.

These changes began out of need, but now I think I like them for their own sake. Being able to sacrifice Pokemon to serve as energy means that nearly every card drawn (from a deck consisting almost entirely of Pokemon cards, in this case) can be put to some use, whereas usually when I play a TCG I run into frustrating and un-fun streaks of cards I can't do anything with in the situation. Plenty of times while playing, I found myself in rather interesting situations where I had to choose whether to hold on to a potentially useful Pokemon or sacrifice it to sate more immediate concerns.