A thoroughly-documented API.

Yea, that wiki is kinda red on quite a lot of the API pages

One central and complete source of documentation for snippet and plugin writers would open ModX right up to many more users. In turn that could help generate more features and even maybe more eyeballs to help tweak the core code to help with the development cycles of existing modules etc.
God knows I wish I used more of the undocumented api's whilst re-writting some of the core php but wasn't able to spend time experimenting to get results.
I understand it's a spare-time resource and y'all are busy on 0.97 but one day I'd love to see the api completely in there and benefit from that awesome functionality that using ModX api's gives.
Cheers
TxRx