ecmascriptif you want to use the new shiny
es5-shimsounds like a good idea too
So what we’re planning to do is to build templating on top of React, and to encourage Blaze users to upgrade to it (or, for those that prefer it, to plain React JSX). We think that this is the best possible “Blaze 2". If you choose to upgrade you’ll have to port your templates, but that’d probably be the case no matter what direction we go with Blaze 2. And your porting effort will be richly rewarded with access to React’s in-browser debugger tools18, vibrant component ecosystem31, and more. Today’s Blaze will continue to work but MDG’s development efforts will be focused on the new React-based templating option, React JSX, and on Angular.
Seriously, don't change the existing API if possible. Not right now. The API isn't perfect, but that doesn't matter. Use the existing API as a baseline; swap out the underlying functionality; and make sure it's feature compatible. It's a classic refactor. It's not as fun and glamorous as producing new functionality or stomping out bugs; but it's the boring, responsible, professional, and trustworthy thing to do. Once Blaze 2 is proven to be feature compatible with Blaze 1, then lets start adding new features in Blaze 2.1 and later.