These are all cool ideas but just a point to make:

The problem with expanding further is payment. Not only will people be working voluntarily, but they would need to take a pay cut because they would no longer be eligible for the curie half (the bigger half) of our votes. It's not easy for people to see the bigger picture of the community in this regard.

Even if this were the case, we have no more VP to give them even the steemSTEM half of rewards so they would basically be working without any extra entitlement to votes, and any vote they get would be a fraction of what it was. This makes it very difficult to find trustworthy, hardworking individuals who stay long term.

We've actually been extremely lucky so far finding the team we have gotten but as things grow, this kind of luck should be expected less. These are reasons why we decline very kind offers from individuals who want to help out; they essentially get punished for contributing to the cause.

Regarding distribution, again because we have a fairly small team, it's broken down quite simply and double screening happens rarely, something I notice perhaps once every week or less. This is because the core curators cover all time zones (me in Asia, JTM & kryzsec in North America and lemouth, trumpman, ruthgirl & zest in Europe/Africa time zones), and we simply cover a period of time available to us.

On top of that our sub-communities such as de-stem and stemng have their own secondary tags which we leave alone similar to your suggestion. I imagine when we are way bigger, secondary subject tags will likely be inevitable, assuming steemit.inc continues to not release a more workable interface.

Thanks for the ideas!