Check Nick's response on this thread:
Customize Notifications
Not sure if you'll be able to include a LINK to the originating idea, just values from the idea attributes (which you are already doing). I find that we have vastly more flexibility with emails when we just use GEL script to create them. We can include all the data we want, format as HTML, and include links to any objects in CA PPM.
I think the issue is that you'll still have this default action item notification being sent out in addition to any GEL script email that you generate. To my knowledge you can't bypass those emails on an ad-hoc basis for just one step in one process. If your users are OK with disregarding the first message and using the GEL script email instead that would be fine. If you really wanted to go nuclear you could disable EVERYONE'S notifications system-wide related to process action items but then you'd have to always send a GEL email every time you have an action item being created.