I highly recommend you consider adding a very simple import/export option to the template modification system so that developers and even non-developer site owners can import/export/share template modifications. I know you can do it with an addon, but that's extreme overkill when only a small modification is necessary.
For example, one of the modifications I share on all of my sites is to make "Stay logged in" default to checked (on). While I understand the security reasoning for it being disabled by default, we were getting way too many "Your site keeps logging me out" complaints. It's a simple one line change obviously and I'm sure someone has made an addon for it, but it's a lot more hassle to go through uploading and importing addons for simple changes.
I personally think that with the TMS, XF has something that could be hugely popular .. a template modification resource library. For the developer/creator of the modification, they just have to export it to a single XML file. Then anyone could import it directly into the TMS basically just as if they manually entered it. No server-side uploading and permission changes required. No separate "imported modifications" section would be required either .. just import it directly as if they manually created the modification.
I understand the desire to feature addons and cater to developers, but the TMS feature is something that could explode in popularity for all XF owners, not just the developers.