I think adding some use for builders once your base is maxed is a good one, and would encourage continued VIP subscription, which is good for SG.
A couple people have proposed ideas similar to yours:
As well as some alternative post-max VIP perks:
And a couple people have discussed/lamented the current lack of use for idle builders:
There’s also a compendium post of threads related to the second builder in general, which has a variety of ideas about improvements to the second builder: