Something where several artist come together for a collaboration like @ryssen said would be nice, and I am not knocking the idea of a collaboration poster to help market FAKKU!, I already said earlier that having some FAKKU! commissioned art on the site would be good marketing.
But what about commissioning something like the 2016 Wani winter special collaboration booklet:
If having 1 artist create a full 200-280 page tank is going to take too long, then maybe a collaboration there would help also.
1 story line, all main character designs set, and then you can have different artist compile the book together, just like how the Bestiary series is from Da Hootch. (I understand the Bestiary series is not actually a story line book, I am just making suggestions.)
Last suggestion, just like Toranoana, and Melonbooks, etc. have a special limited set with FAKKU! store exclusive purchase. Just like in the name it's limited you have a small batch printed and sold with the tabkobons at a premium price.
Most people already know this but, It's a 4 page booklet, double sided so 8 illustrated pages.
Edit: if I could suggest any artist for this it would be Key, Key (to my knowledge) has no social media contacts, and Key (to my knowledge) doesn't prepare any Doujinshi for Comiket.
In that regarding commissioning Key to make some original Doujinshi, or a book for FAKKU! Would be extra special.