as a wargamer I have found the secondary sets less appealing than the main theme in the last 3 KSers.
Went big in CBS then even bigger in Castles (did you know I like castles?)
Did not get any sewers in KS3 and so far have not regretted that decision.
I did got for a complete set of mountains in KS4 as I could see using them and then got another Erinthor Mountain set at retail once I saw how great they looked.
Surprisingly - no burrows for me in KSV - they look great but like the sewers they don't lend themselves to war gamming. Having said that some Aliens v Predators v Marines skirmish action would be very effective in a Sci-Fi burrows variation.
Future secondary themes? most have been mentioned by Berny
ports, cathedrals (big buildings), moats though these are sort of covered with water caverns and terrain trays but I'd still love to see a dedicated moat with curved pieces to around both 6" and 4" towers - should be able to double up as a mountains compatible river.
What I would love to see is Ruins and made this point during KS4 viewtopic.php?f=47&t=7790
I think they make a perfect secondary theme, ruins are timeless, exist everywhere and are perfect for both RPG and war gamming.
One last thing - I know Stefan is planning a minis KS, I'd love to see a whole market setup - stalls, merchants, customers, thieves beggars etc with a lot of stalls (burgers, hog roast, brass, cloth, weapons, silks, jewellery, veg, meat, ..... think Ankh-Morpork, but with out the smell.....unless you provide your own ......