Level the playing field; make everyone have an equal spending for equal benefits.
Huh?
Take High Command. It's like a subscription, everyone that has it pays the same.
Now, this idea comes from a different browser game called Tribal Wars (TW). It has a free version, that most new players are using. The game is fully operational, and all features (building, moving, fighting, everything) is available. However, the game is about empire-building and getting hundreds of villages. The free version allows you to click every village, see what it's doing, and adjust it (like, build something).
When you have hundreds of villages, this becomes an awful job. You have no idea what is going on in your empire, the villages don't issue any kind of warning like "I finished something, and I am idle now."
This is where the subscription comes in. If you pay a monthly fee, several handy overviews become available. Like, a list of all villages, and what they're doing, how far they are with that, what their status is, etcetera. There are several more, but lets focus on this one. Because it sounds familiar, right?
CoW has a "Province Overview" that does exactly that. It is free for everyone. You don't really need it when you are just starting out (you just have 4 or 5 cities, and you know perfectly well what they are doing and when they're done); but when your empire grows, it becomes indispensible.
For TW this meant, that all players who "counted in the world" payed the subscription fee (iirc it was about € 20 a month); the free version was only used by new players, until their empire grew to a level where it wasn't feasible to handle it anymore (or dropped out of the game, of course). Company happy, players happy.
I can easily imagine a similar subscription for CoW. If the province feature isn't "enough" incentive to get people to pay (and on the smaller maps that is well possible), some more features could be disabled:
- Zooming the map;
- Filtering the World Herald;
- "Add destination" feature for unit movement;
- leaving this thread here will generate many more ideas in less than a day.
Of course, the non-paying players should often get a "free-for-a-few-days" version, so they can feel what they are missing when it is taken away again... and I'm pretty sure that many of them would decide that it's actually worth it.
In the end, TW became the only browser game I ever payed for. The "Unlimited Gold" system tastes very bad in my mouth, and I don't want "real" in-game advantages over my fellow players, because a win wouldn't feel like a win anymore. But a subscription with the ONLY advantage making the actual gameplay (and thus "my life") easier, yeah, I'd pay for that. And I'm pretty sure that goes for a huge number of the player base.