A single setup assistant to rule them all.
This new setup assistant tracks and manges all the versions of the engine installed.
Just one version of every management tool installed " the latest " able to work older engine installations ( you may start with 1.11 tools being able to manage 1.10 projects maybe too late for 1.9).
Place third party dependencies in a central place and not with the specific engine version so the tool is being able to not store double dependencies.
Make able in the tool to download just one dependency so waiting for the shared feature user may use symlinks to the old engine directory when the version hasn’t changed this can be just possible with a more strict workflow(you need to make the simlinks first).
Some kind of strategy to manage multiple gem library locations and manage multiple versions of the same gem(the tool may choose what version to add to a project).
Locally cached gem compiled assets with symlinks in game cache directory.