Editing Warhammer 40,000/8th Edition Tactics/Imperial Knights (section)

===Space Marines===
Well, you've probably got some lying around, so that's a start. Oathsworn Knights can often be found accompanying the marines into battle, though fluffwise it's very much on the side of Marines supported by Knights rather than Knights supported by Marines. The downside is that marines (the dudes) are somewhat pricey, and tactical marines are rather uninspiring (even if this edition has made them better). Marines have good and cost-effective tanks, though, so that's always an option; you'll find it hard to go wrong with Razorspam. Librarians are a no-brainer for your HQ; it'll be nice to actually have something to do during the psychic phase for a change, even if their powers are shit (unless you take a vanguard librarian, in which case have fun with your newfound cheese).

However, with the August 2019 Codex, you’ll be severely disadvantaging your marines with the loss of Combat Doctrines by including them in an army with non-marines.

You could always just grab Guilliman on his own or with a posse of Librarians and Smash Captains. The loss of doctrines won't hurt these units so much as they aren't so reliant on them in the first place. Beware, Knights + Guilliman is pretty cheesy.