After discussing this with psydev, we came up with the following - subject to change
* Add a maximum build range to CY - start with 10 cells and adjust as needed.
* Existing building-spacing rules still apply within the maximum radius.
* Any production queue can place buildings around any CY.
* Placing a structure starts a cooldown timer on the CY associated with the placement region. Additional structures cannot be placed in the same area until the timer expires (but can be placed around other CYs).
* Cooldown timer period is set to equal or less than the fastest build time, so this will not impact players with a single queue. Start with 8s and adjust as needed.
* Cooldown timer also applies when placing structures around an allied CY, so team-building can hinder your ally if you do it without proper communication.
* Cooldown progress is displayed either with a selection bar, a timer overlay on the CY (like the repair icon), or by shading the build-radius ring to display as a giant clock. Indicators are visible by the player and their allies.
* Engineer capture delay removed to counter not being able to build off captures.
* (not discussed) Potential to add a Mobile HQ as a secondary base provider without a construction queue, for early expansion and setting up defenses around tech buildings. Construction range would be less than a regular CY.