If you don’t have any answer and still want to build, you could build for any device for which the last officially LineageOS supported version is Nougat AND amoung Sony, Xiaomi, YU or ZTE brand. (Because everything else has already been done.)
Here is the list of codenames that remains to do :
Sony :
taoshan
nicki
ivy
karin
karin_windy
Xiaomi :
hydrogen
armani (Out of space)
ido
YU :
tomato
ZTE :
jasmine
tulip
Thanks (even if you don’t do it 
)