I am super excited for this, I played hundreds if not thousands of hours of sins, mostly multiplayer. My multiplayer experience was marred somewhat by z-axis movement cheese which broke some aspects of the game such as making non red button stationary starbases + mines literally useless and enabling vasari starbase rush abuse.
I am keen to see a little more rubberbanding which could enable players who have lost a levelled capital to phase missile focus or who were a little too slow to respond to a corvette swarm a way to survive in the game. Often games were over in the first hour due to one player making a small mistake with no hope of a comeback. AOE solves this with the town center, a building which provides a small amount of home turf advantage.
Specifically I would like to see stationary defences be far more useful and I think this would address my concern. Most stationary defences had sufficient offensive power in the first sins however could be trivially bypassed or lacked enough range to cover any strategically useful areas. Typically your defences were built in the mid to late game and served to augment your army on the frontline. If your army was out of position or weak they were functionally useless. If most defences had a long range or limited movement within an area this would give players more options to respond to aggression.
I hope the directional weapons and the scaling damage reduction mechanic that shields used makes a comeback as that was awesome and made fleet control and focus fire decisions skill intensive and very interesting.
z axis cheese? I did not see this in sins1 but was disappointed that the Z axis was not able to be used more. I think Starbases should be key to defending orbiting facilities not the planet. Additionally non-warp capable ships should be in system and cheaper than the warp capable ships (think real life naval monitors with BB guns) to head off incoming fleets.
Also think that starbases should have one time thrusters that allow reasonably fast redeployment to the area being attacked. Fast enough that any bombardment craft could not clear off before it arrives.
To defend a planet ground based planetary defences should be a feature you can build in hexes all over the planets surface. Surface to space rail guns or lasers and such?? This way much more of the planet could be covered
Orbital mines and bombers are already a thing, but perhaps heavy bombers should be considered as well.
Late game you should have orbital shields. Of course you have the garrison ships auto build skill as well.
In short I do not think the solution to defending a planet should be defending phase lanes and precluding any Z axis space movement. Space is 3D after all and this is what is different about any good space game from it's land based counterparts. Of course barely any games do Z axis assuming everything happens on a land based X,Y plane.
I do wish we had armies in game but we only have navies (space navies).
As to being able to defend everything at once, that is down to game balance and whether you have fleets stationed withing a jump or two. Sins of course suffers from the mega blob tendency by players of putting all their ships in one fleet to be able to make any meaningful progress. That is on the devs and is all balance.