First of all, the idea of the select_clause and the from_clause is great. Contacts has been doing this for more than two years now and it works great.
What we also do is have a callback which allows other modules to register what extension columns they provide for this list. As I use the same technique for the generic object_type list (it's an include which allows you to get all objects of an object_type in the list, with the ability to add and remove columns as you see fit. It also works with relationship object types, displaying both object_id_one and object_id_two with the extension columns for both) I would love to see this method made easier by the TIP (currently the code for the object_type list is over 650 lines long, too much for my liking).
Just mentioning it, not sure if this has any effect on your decision what to TIP, but the parallel TIP sounds a little bit like it would make my code shorter.
I presume you will document those new features properly in the docs for template::list::create ?
