Creating partials outside of the templates folder
So a few potential things to consider:


You do not want to be serving static assets like Javascript, CSS, etc. out of the templates directory.  I realize you're planning to put them "elsewhere", but ensure that elsewhere is in the /public/ folder, so that php, Yii, and Craft do not have to be spun up just to serve a static asset.  Do not use a Craft routine/plugin to load them.
The entire point of modularization is that the payoff is down the road as you're able to reuse components on future projects by just "plugging them in".  I've seen this approach taken on many types of software projects, from desktop apps to mobile apps to websites.  It almost never works out as well as people think it will, unless it is seriously well architected, because we're generally not a factory churning out widgets.  There are things unique to each website beyond the basic functionality that requires customization; writing something that is both that general, and also useful is difficult.  Think of how many hours you have spent fighting even well-implemented frameworks like Bootstrap or Foundation.
Most things of this nature either end up being way, way over engineered and bloated, with a ton of time spent on them, and the promised gains down the road in terms of time savings never being realized due to the aforementioned issues.


Just my two cents.