You can already do that with .xml, though you can't create a mod that lets you make other mods with .xml, like as you mentioned a mod that lets you customise the tech tree. That would need to be done with a tool that's been specifically programmed and would likely take a professional programmer to do.
You may be able to make life a little easier for yourself by making a spreadsheet or excel doc, with the branch's set out with references to their associated location within the xml. files. This makes it easier in the long run but it would be a lot of work to set up fully for all techs of all factions. I have a partial set up like this for a couple of branch's of only 1 faction. I have been thinking about doing it fully but am waiting to see what SD has already made and said they will share at some stage, which will most likely be a lot more comprehensive.
