this has certainly been discussed by the devs.   It is likely going to be a while before a lot of custom things make it in, but in theory players should be able to choose which techs are or are not allowed in, rearrange tech trees as they see fit, and reprogram or adjust the effects of techs.   Some goes for spells.    The single player story mode might not be able to be adjusted, but for skirmish and multiplayer games you can play it any way you want.   If you don't like a certain spell or tech, you are eventually going to be able to choose not to include it, and rebuild the tech tree around its absence if needed.