Reading through the posts in this thread, it seems to be turning into a situation where people are asking for 0 A.D. to copy some things from AoE 4.
The FSF recommends that free software projects avoid competing directly with proprietary software, because that would be playing to the strength of the proprietary software companies (money). Instead, compete in ways that FLOSS software is strong, such as the license privileges that users and volunteer developers have, the modability, the community and the long-term continuation of the project (in contrast to Microsoft's "end of life" policy that discontinues support for old software).
Also, something that AoE 4 lacks, I would encourage meaningful improvements to realism and military simulation. This would include things like the following.
Walls block arrows and javelins (except, of course, when fired through firing ports)
Soldiers garrisoned on top of open stone walls with battlements have better protection from attacks
Realistic castle defensive weapons are simulated like boiliing oil and fire pots for fire arrows fired by defenders.
Concealment and camouflage are simulated so that ambushes and guerrilla warfare are possible. Okay, AoE 4 does provide this feature, based on gameplay preview videos.
Battering rams aren't autonomous. They require drivers, who are vulnerable to boiling oil, fire, spears, etc.)
Realism elements from the ancient history of warfare and castle fortifications, defenses and siege weaponry are added to the game. Dakara's suggestion is on the right track here with "more interaction with the ground (trap, bridge, fire at tree, stone wall and roman wood wall...)"
A line of melee troops can block enemies from moving through them. Currently an enemy can walk right through a line of spearmen and enter a structure in order to garrison it. This is not realistic.
In my opinion, anything that is disrupting the application of real ancient warfare tactics and strategy should be considered for improvements to the game. That way 0 A.D. would end up with even more of a learning benefit for players, as well as become useful as a wargaming experimentation platform. And it would advance the art of war gaming instead of regressing into direct competition with Microsoft.
In a way, 0 A.D. will be competing with AoE 4 for players, but what we need to emphasize is that it is not an apples-to-apples comparison. The more knoledgeable that a gamer is about software freedom, the more that the gamer will see that 0 A.D. is an orange instead of an apple.