I would like to be able to have some changes in Assumptions, e.g. climate sensitivity, progress rations, shown the BAU graph, so that subsequent policy lever shifts are relative to those modified baselines. Is that possible to add?

This is a bit tricky. I assume that most people want to vary an Assumption (like climate sensitivity) to see how it compares to BAU.  If we changed BAU, they there is no baseline for comparison. 

My instinct is that this current behavior is vital for understanding.

Given that, do you have a suggestion about how we could add your feature without too much complexity?  

I'd be happy to think with you about possible implementations.  

Curious if anyone else sees value in this? 

A few thoughts:

1. I think there is a difference between varying assumptions and moving the "policy" sliders. The former relate to how we think the world works; the latter are about actions we wish to take. This is already considered in En-Roads given that they are dealt with separately on the interface. However, it does raise a few issues: 1) users cannot currently tell if the changes they see in the "Current Scenario" are a result of changing assumptions or changing policies and 2) it is likely that the policy actions chosen (and their effects) will differ depending upon what the BAU (keeping all the policy sliders at their status quo values, but changing some of the assumptions) looks like.
2. If the effects of changing assumptions and changing policies were simply additive, you could just add a third line to the graphs showing both the BAU and Current Scenario. This third line could be called "Modified BAU" and the original line called "Default BAU". Then the user could see the added effect of their policy choices in the Current Scenario line. Given that the effects are unlikely to be additive, though, it would really be best to be able to add two lines, giving 4 lines: 1)Default BAU, 2)Current Scenario under Default BAU, 3)Modified BAU, and 4)Current Scenario under Modified BAU. Obviously, this complication could work against the goals of not making the graphs too complicated. It also requires the software store more results to make the additional graphs. Finally, it also leaves open the question of what to show on those graphs that already have multiple lines. I assume they would reflect the Current Scenario under Modified BAU results.
3. A related query would be whether it would be possible to show the results for the BAU and Current Scenario, plus a saved Scenario on a single graph. If I remember correctly, though, En-Roads does not currently let you save scenarios, which would require extra memory in the software.
4. Perhaps all of these options are something to consider for an eventual, slightly more advanced version of the model that would not necessarily work on the web.