Yes... in the new mechanic there are no real defender or attacker, there are just simply fighting.
Which is a good thing. Since at least WW1 (perhaps even earlier but I'm not an expert in early modern warfare) battles have not been strictly about an attacker and a defender. Popular culture and pop-history still uses those terms liberally despite it being very misleading. Even in conflicts as ludicrously lopsided as the 2003 Iraq invasion by USA, Iraqis did perform a handful of counter-attacks, and if you go down to the tactical level, almost all modern battles are a mixture of attack and defence.
I think there perhaps should be a clear attacker and defender in any given moment.
I disagree. Aurora ground combat is still very abstract. It's certainly not simulation-lite style like space combat is. It's not a problem that both sides can remain fortified because the fortifications are just an abstract concept of how well the force utilises both natural and constructed features of the planet they are on. Remember, even millions and millions of troops would only use a tiny little blip of a planet's surface.
the point was that since an attacker's units can keep their fortification bonus and still fight, there isn't any unit that's better on the defense than the offense.
This isn't really a problem either. In VB6, the divide between attack strength and defence strength was always ridiculous and extremely difficult to justify from "realism" or even "story" perspective. How can a garrison unit be decent on defence but have literally zero attack ability? How can mobile infantry be better at defending than attacking, when projectile weapons are equally lethal/powerful regardless of circumstances? We had to come up with all sort of silly justifications for why assault infantry was good at assault but bad at defence and so on and so forth, or just ignore the whole thing, as evidence by many player AARs here that gloss over ground combat.
You still had Garrison units that were for those situations when you needed to defend rather than attack, having a big number of those cheap units made it possible for a weaker side to properly defend themselves cheaply.
But just defending would not end the war and the stronger power could just as easily spam garrison themselves and since the weaker powers could not join forces, the conclusion was always the same and the only variable was the length of the ground war - eventually the stronger power would win. At least C# fixes this by allowing the weaker powers to join forces as I said, meaning that multi-faction starts on Earth will have a modicum of balance by default if you're going with 3+ factions.
Let's say you play an Earth multi faction game and two factions want to fight a limited war over some colonies. They don't want to start WW3 back on Earth for the control of a couple of million people in the colonies. There should basically be three engagement levels, offensive, defensive and stand down mode on each world.
This is an entirely different thing and I agree. It was asked for earlier, I recall. While it's a fairly niche thing, it would be very useful for multi-faction Earth starts, to have a situation where even the rear support formations would not engage other powers.
Remember that:
Ground forces can be assigned one of four field positions; front line attack, front line defence, support and rear echelon. Units in support and rear echelon positions cannot directly attack hostile forces but if they possess elements with bombardment weapons they may be assigned to support a front line formation. Support and rear echelon formations can also potentially provide anti-air cover (more in a rules post on ground-space interaction) and supply to front line units. Only formations with all elements supplied can be placed in front line attack mode. Formations placed in front line attack mode lose any fortification bonus.
So if both sides have all units in support or rear echelon, there is no combat taking place since there are no front line units to utilise that support. And if both sides put all their units in front line defence, then there is only bombardment attacks. If one side attacks, then the attacking units cannot use fortifications.
So yeah, it is useful for attacker to first fortify as much as possible, to minimise casualties from defender bombardment, but if they want to capture the planet, they can't just hide in their fortifications.