You could add Twig into Fuel in the time it took you to write that comment.
To answer your question specifically: it would be smart to use Twig by default, rather than developing your own custom Fuel template engine, because you would need to spend a significant amount of time to get something with the same feature-set, build quality and documentation coverage. Would this time not be better spent elsewhere?
As a pretty content Fuel user, I should also mention that your tone doesn't benefit the project.
Your replies to any not purely positive comments or criticism in this thread or elsewhere come off as very abrasive and rude by the way: not the kind of attitude HN expects and is certainly not the way to attract more of a community.
Your framework looks very neat and I'm interested in seeing what it turns into in the coming months/years: don't spoil your reputation before you even leave the gate.