First Foundings is cool, but after it I would like to see a book exclusively focused on other chapters.
-The Mantis Warriors and their Dust Prophets, a chapter which always worked closely with the Deathwatch.
-The Minotaurs, a chapter which has focused almost exclusively on fighting other space marines.
-The Astral Claws, at this time believed to be one of the most pure and noble of space marine chapters
-The nearly unknown Carcharodons and Star Phantoms.
-The dour Red Scorpions and Charnel Guard
-Dare I say it... The Rainbow Warriors.
And literally a thousand other options.