Thanks for the suggestions guys.
I had previously realised that if your capital was at maximum population then bulding a pioneer was a good idea. However I had not realised that you could reduce population further by queuing them up. Presumably a relatively simple fix would be to only pay the cost for the building when you actually start building it, as opposed to when you queue it up. I guess that complicates things if you've queued up things you don't have the resources for when you should start building them, but having a check and then cancelling the build if you don't have the resources (and giving you a message) should be possible.
I wouldn't mind if they looked at population in more detail in a future release, it seems a slightly odd mechanic, where (for example) you can reduce population to 1 and have no effect on tax or production.
I believe they're looking at staffs in the next patch, mainly to make them more useful for characters. Magic damage for trained troops is powerful, but in theory there are counters by using cloaks, spells, etc.
I agree it's not very often that you find a three essence city site, let alone four essence. But I still don't think you should get a +1 bonus per essence. I've generally found a three essence city at some point fairly early on. Having +1 initiative, +1 fire damage, +1 hit points is pretty powerful. Having +3 initiative, and +3 fire damage, and +3 hit points, just seems ludicrously overpowered, let alone if you have four essence. I did wonder if you should be allowed to cast the same spell multiple times on the same city, so in other words you could still get+3 fire damage if you were prepared to cast heart of fire three times, just not with hit points and initiative as well. However that would presumably be a more complicated change than just making the bonus +1 rather than +1 per essence.