One thing in Telegraf where I didn't figure out a good solution was a way to parse arbitrary log files and generate data points and/or annotations from them.
There is a particularly annoying log file format from a proprietary application containing data I like to monitor which contains time series values in a multiline format as well as error messages. What I'd like to do is to have Telegraf tail the log file and pass it through a script that generates actual influxdb data from that. So something similar like the telegraf tail plugin, but with a data transformation in between.