Well, to be honest I've put a bit of thought into this. See here's how it would work in my eyes.
1) Get rid of ALL starting influence (even the 25 from your very first city)
2) Drop the gain rate of influence instead of 2-6 make it an unshakable 1 per quest 2-5 per pvp battle (obviously more if dragons are involved and dying) - to promote pvp
So yes, in theory you could just keep creating multiple starting cities and then you could transfer that to your main city. But that's still making the individual put a lot of time into it, like now I just finished building my walls. Basically the only thing I was logging in for was to trade food for stone. If I had more incentive to start another city now, like to better the power of my main city I would do so.
Basically yes, shared influence would make things easier, at the same time I think the influence gain should be reduced. If that makes sense, so it's not absurdly overpowered and you still have to work for it, but it's also conveniently there to put in any city you desire. Basically like a shared bank between all cities, with low influence gain so you're putting in the time and effort whether you're playing your main city or an alt city.