At level 15, all further progress is done through specialisations, such as "trapmaking" or "civil engineering". Multiple specialisations can be pursued, but must be paid for separately and do not stack.
We're still figuring out how much a given number of dice gets you in terms of things you can create/achieve, and would like to invite suggestions from you guys about what different levels of Mechanical Aptitude should be able to do.
Let me suggest some specializations and stuff that might be done with them.
Weaponcrafting - At highest levels allows you to work with chakra-active material and craft weapons that can channel chakra. At lower levels allows you to produce reinforced weapons personally designed and weighted for an individual combat style and able to take the rigor of ninja combat.
Traps - Not sure where to go other than "harder to spot" and "harder to avoid".
Radio Technology - The principles of radio actually have been discovered but crafting remote communication devices is a closely held secret and each unit must be hand-built along with its batteries. At lower levels units are backpack-size or bigger... only "portable" because ninja have superhuman strength. At max level you can custom-build headset radios that allow an entire team to coordinate while spread out over a forest. (QMs might veto this one, but I thought I'd try it.)
Architecture - Incorporate traps, hidden passages, and choke points into a building. Also handles lock design and lock-picking. And designing buildings, of course.
Puppetry - Used to build combat puppets that can be controlled with chakra strings.
Clockwork - Used to create clockwork mechanics. Primarily used for constructing clocks and watches that allow coordination down to the second and make navigation much easier at night and over sea. At high levels can construct clockwork-powered tools that are able to move and act on their own to some extent.
[jk]
Giant Moon Destroying Death Lasers - Used to construct giant lasers that can destroy the moon. Of course that's at the highest level. You have to start small with lasers able to destroy only smaller asteroids.