The existing add on chimney sets are very nice but some bigger ones, the same width as the lower part of the normal house kit chimneys would be very handy for scratch building. At the moment I saw off these as low as I reasonably can from your house kits so I can stick (magnetically) the smaller diameter bit on top and pinch the rest for my own.

Of course for Stalingrad and wild west films the chimney stack was often the only part left when an otherwise wooden house burnt down so complete chimneys would be useful.

I add chimney pots from a bit of plastic tube pegged on so caps with a round hole are the best, I don't have to faff about drilling them.