[SDPhantom]

In writing a new client from scratch, a challenge would be trying to develop a specification to use to detect an addon's version from the TOC file. Curse uses custom metatags in the TOC to operate their client. In this process, actually writing up the specification and all the technical details is easy, the hard part is getting everyone to use it so the client would actually work.

I think one design spec of Minion was the ability to configure it to reach across to other sites, like Curse, and look for addons over there too.

An alternative to creating a download client is to create an updater library for Lua that addons would register to. It would communicate to copies of itself in the addon channel and if it finds a registered addon with a version higher than yours, it'll alert you to check for the newer version. This is a feature already built-in to some addons like DeadlyBossMods.