You already have that option, and it's on by default when you install the game. 

It is a major feature, and there are posts discussing it from A23 era, so it should have been implemented, or at least been a mod, by now.

As I said before, formations should just be turned into battalions, with a bit of Warzone 2100 unit grouping added.

I'm not the one who proposed adding battalions*, but the benefits from my point of view are: - streamlining formations, allowing us to tie formations to battalions, - implementation of battalion-specific bonuses for units that can form a battalion, - de-coupling gatherers from front-line units. In the long run, this will enable us to do all kinds of "battalion-specific" combat roles, allowing players to choose between stronger army or stronger economy. The units that are gathering resources obviously won't belong to any battalion, and they will, for example, need to drop-off resources in order to form battalions. Furthermore, individual units will be weaker than units in a battalion, - more opportunities for strategic positioning, decisions on when to attack and with what, etc. - implementation of a "shared experience pool" between units in a battalion. All units in a battalion will share the combat experience, and when an unit from a battalion dies, its experience is shared between the surviving soldiers. Cons would be the massive changes to the game's meta, and the cost of implementing all this.

@Classic-Burger I can explain why a sheet with a bunch of ideas doesn’t translate to an equivalent bunch of gameplay changes/features. gameplay changes should fit into the scope and style of 0ad, be compatible with existing features, not introduce unnecessary complication, while enriching gameplay. For example, users may suggest realism features, like capturing wild horses to give the player a 1-time discount on a cavalryman. However, that would conflict with other features, like siege speed, hero HP, as 0ad is not an exhaustive simulation. if every idea we came across was implemented as is, 0ad would be quite a mess, wouldn’t it? One other thing is that these changes require people’s work to get them over the finish line. So arguments for a new feature should either convince a dev to take up the task, or the arguer should try it themselves. And that means you may need to modify or walk back the original idea to get more people in agreement.