I'm always advocating for things that increase variety, and one thing that could go a long way for that is would be to introduce substitution to shipping containers. Currently we have two types of CSX container for sandpatch, and two containers (Hamburg-Sud and haupaug Lloyd) from Rhine-Ruhr Osten. Why not have these cross polinate? if the HS and HL containers could appear on sandpatch, that would make for 16 possible double stack combintions. Add in container livery designer on top of that, you could bring that up to at least 24, really upping the variety and flavor of intermodal trains.
Yes I do think that would be great but as you gave an example, CSX might not be happy seeing another companies Containers on their trains- for livery designer it would be nice but you would need a bit of a "consist designer" to make it work nicely
I am fairly certian that the game already randomly chooses between the two types of shipping container for the well cars as it is, all the livery designer would need to do is slip your designs into the pool for that choice on generation of the consist.