I'm working on a Ubuntu based distro with some custom packages, I want to insert a step in which I give the user ability to select the desktop environment and packages he wants to install. I know the installer is Ubiquity (derivative od d-i) but from where can I add the steps?
Based on the following I would guess one could modify the d-i code to change the behavior.
"Where d-i code is used, it is generally possible to preseed it to change default behaviour. See the installation guide for details. By default, preseeding is confined to setting default values for questions, and Ubiquity will not skip over preseeded steps (this is to allow sensible interpretation of going backward and forward in the UI); if the --automatic option is used, preseeded steps will be skipped."
There is also a q&a page on launchpad that may help.