Seperate Queue for Units

I'm proposing a separate queue for units, which means you'd be able to build structures and units at the same time in a single city. Having a separate queue for units has a lot of benefits and less negatives than you'd think. 

Balancing

The way creating units works in Humankind is every unit costs population. By making a city's output more reliant on population that basically fixes the balance problem that would be created by making the queues separate. You can't keep making units because with zero population you'd have very little production which means you can't build structures/gain science or gold.

Additionally, units have an upkeep cost which means you can't keep creating a bigger army because it becomes more and more expensive.

Benefits

It fixes deficiency problems for military focused cultures. A military culture can easily fall behind when it's only making military units every turn instead of investing in science and production. Usually, they make up for this by conquering other cities but if they don't succeed, they lose their entire investment (their army) and are now massively behind. A separate queue fixes this problem by letting them create their army while also building infrastructure albeit at a slower rate because of less population.

It also fixes defense problems for non-military cultures and makes games more fun for them. Most of these kinds of cultures don't make an army until they are attacked. Which means they miss out on a big part of the game before that time. An is not just for offense and defense it's also for exploration, hunting animals and small skirmishes. It's a fun part of the game which non-military cultures often don't partake in because they'd rather focus on their structures. A separate queue fixes this problem by making building units more accessible because they won't be totally halting their intended strategy to produce units.

It would fix naval warfare. Amplitude is trying to fix naval warfare by improving naval warfare. The way I see it it's not a naval warfare problem, it's a production problem. You can only produce one thing in a city at a time, which means you always pick the most cost-effective option. For warfare land units are almost always superior simply because they're more flexible. A separate queue vastly improves the production problem because now you're no longer choosing between land units, naval units, districts and structures, but instead just land and naval units.

Cultural balance changes

A separate unit queue gives agricultural cultures a much-needed boost without making them overpowered because you still need money for unit upkeep. It's also a nerf for gold focused cultures because they were previously the only ones able to make structures and units at the same time by simply purchasing them. Now everyone can do it without needing to be a millionaire.

Negatives

As I hope you've now realized units in Humankind are already well balanced enough for this change to happen. But one negative I can still think of is player incompetence. Incompetent players/noobs might always be creating units just because they think the queue should never be empty causing their population and gold to drain without them realizing why. A disclaimer with a short explanation of the cost of units should be sufficient