There are many reasons for JAPONESQUE low sales, and I agree with all your ideas. Especially what ayu_love said about her formula for albums, which imo is being the same since secret.
I dream of Kumi making a temporary unite with a band [BACK-ON would be the best one for many reasons]. This would add much more to her style and make her grow as an artist. Her voice is getting more powerful through the years, so I think that giving a bigger attention to her rockish side would be the best bet that they could do, and looks like she's already giving a try to it, looking at BBBoys, No Man's Land and even Poppin' love cocktail you can see a bit of it.
Also, I think she's aware of this need of changes, and now that she's completed 10 studio albums, she will renew a bit her "Koda Kumi" image. She feels she needs to change, and as a proof she hasn't released a summer song like Lick me and 674 other similar tracks since 2010. I think she said it on a magazine interview, that she felt she should change her summer tracks style, and both tries [Lollipop and PLC] were quite successful. Next step is changing her ballads style now hahaha
I'd love Kuu to release some songs like No way to say by ayu or Believe by Maki Goto.