Z-axis would be a problem to keep, because FOR EXAMPLE how would you make turrets more useful then? They would be totally useless as they are.
Increase their range? That would defeat the purpose of LONG range frigates, which now outrange turrets (and that was one of their roles), leaving you with carrier spam.
You'd have to increase the range of LRFs in return, then Starbase's (because LRFs would be outranging SBs now).
Effect - everything shoots through the whole gravwell. Gravwells will no longer be battlefields that allow tactical movement, but empty wild west streets at noon with two fleets standing in front of each other with nowhere to hide, hoping to kill the enemy faster than he kills you.
You'd be jumping from one boxing ring to another. Meh.
Increase the size of gravwell to fix this? You would just loop the problem back to the first point. Only that all ships would now be half size and look like ants. AND fleets would take FOREVER to pass gravwells.
Vertical Cluster turrets? Nah. How would you destroy them? Say, we have three turrets vertically. They still have to be worth 1 tac slot, otherwise they aren't worth it. Each of them still has to be as tough as it is, otherwise they're useless (cause it's not all three that fire at one passing fleet).
Would they all go down when one is killed? Or not? If yes, it would be silly. If not, how would you replace 1 or 2 of them? By repairing debris which then reshapes into a turret? Or by placing another cluster of turrets? Would 1 of 3 turrets take 1 tac slot still, or 0.3333 tac slot? If 0.3333.. then one could put 30 turrets in one place by getting rid of the outer ones repatedly.
No, I don't like this idea, sorry. Too fouled up to make it right.
All that just to fix turrets. What about repair bays, SBs, nano jammers. All of them and more would lose their effectiveness if the enemy had a whole new dimension to maneuver around.
Not to mention that mines would become absolutely useless. In homeworld mines were free to place and they still were marginally used, mostly by AI.
No, I believe Sins would require a complete rework with many fundamental gameplay rules changed to fit full 3D.
If you just give Z-axis , some units will simply become useless unless you remake them completely, which will start a chain of other required fixes and Sins will end up as Hegemonia: Legions of Iron, a game that was dead a week after release.
Or as another Homeworld ripoff.
Z axis = can of worms.
Stay away.
PS. If IC is having troubles with balancing LRFs/Carriers over the past ... year(?), then I guess we shouldn't be so sure they would balance the game with Z-axis before the third world war.