This is a pretty important question.
If it also uses fire energy then you have a splashable dark-type attacker in fire Charizard decks (with the inevitable fire-type Charizard ex and a new no rulebox Charizard). If it uses dark energy then suddenly it is best used as its own deck or used in dark decks which is a lot less exciting. Of course if the attack cost is only 1 dark energy and the rest colorless (or even fire), you can try to make a dual type Charizard deck work, but it'll be a bit clunky.
You also gotta wonder how many more types of Charizard ex we'll get, and if we'll get a strong new no rulebox Charizard to go with them (especially with the Vivid Voltage one rotating, and no offense to the GO one which while not bad I have doubts about it seeing play even if we get a good Charizard ex).