I will point out the FW items were resin and had plenty of little features like Blip mentions. But we're not here to recreate and ape FW. We're here to support you in making top of the line items!
I will state agreement with Blip. Those little extras will move this from a nice and usuable though very plain item to a must-have and really will see them sell I expect. Right now as they stand your value proposition is that they are a time saver for people that don't have the time to waste with some plasticard and need something good enough. Let's BAM it up now!

So some interesting small things you can do now:
-Crenelations. Add that as a variation to them (or better a glue on part)
-Create another straight run with a small integrated covered pillbox
-Destroyed versions of the sections
-Retaining Supports on the outside of the walls. It'll break up the flat surface and give them an interesting look.
a. This can be easily done with some .25 mm strips going horizontally and/or vertical strips to look like banding to reinforce the concrete
b. Along the base instead to make it look like a foundation which will, again, make them look a little more visually interesting
c. A diagonal support (triangle) that is dead simple
(editorial note: As they stand now the very first thing I'd add would be I-beams as supports if I purchased these as is but they probably wouldn't cast I expect)
-Add some interesting
kit thrown in on the defenses on the inside. A few small crates, firing steps, tech bling or comunicators would be nice. Even better: sell them as extras that people could purchase to throw on there themselves!

PS That little short 45 degree sloted corner is awesome! Good idea!
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