Duncan doesn't have any affinities, he can get along with everyone. Crreessa is obviously fire. 
Alan is tricky. But since dwarves don't like elves and Gnol can sense Alan is half-elf eventhough his appearance is human I'd say Elves are Air. Making Alan air or at least partly air. His ground dragon doesn't mind in the least or is also part air

Speaking of dwarves. Calle introduced steampunk dwarves, a concept I'll happily work with. I was thinking that besides the normale ways to make steam (wood and coal and stuff for fires) I would like to introduce a special stone that can be charged by dragons. They call these stones Dragon Heartstones. If a fire dragon breathes his fire on one of those stones it will heat like a coal for a very long time and thus becoming a very duarable way to make steam but those stones are very rare. Even the size of pebbles are very hard to come by and it is a well kept secret between the dwarves and dragons. They would need very special container to carry the stones of course. 
This could be extended to the stone absorbing any special trait of the dragon. If a dragon would breath ice over it for example it could be a freezing cold stone for days. If a light dragon puts light on it then it would illuminate for days etc.