Allowing template customizations in a controlled way that doesn't require a fork would be a great improvement. Maybe splitting up packages like you describe is a good approach to achieving this, I don't know. I think I would need a working example or a more detailed description to have an opinion on whether this is the direction for OpenACS to move in. If you could draft the change in one package and show the advantages that would be very helpful.
A different, and more radical, approach to solving the presentation customizability problem is the work that Lars and Branimir are doing, labeled .KUL, aka data soup. Their approach is that you specify in an admin interface which object types you want, which attributes and templates they should use, and which pages your data should show up on. The idea is that you should be able to combine object types with views in a multitude of ways, i.e. show forum posts in a blog view etc.
