I suggest you head instead to the
Background sub-forum.
You can also find inspiration on
1d4chan.
Now I'm not the most knowledgeable lore guy so if they had a home world how would I go about choosing a planet and system? Or would a fleet based chapter make more sense?
If planet-based, I suggest you make up your own planet. There are about a million worlds in the Imperium.
Fleet-based does make a lot of sense for a nomadic chapter. Perhaps their planet was destroyed (e.g. devoured by the Cicatrix Maledictum), killing all the Space Marines at home and forcing those on campaigns to regroup and rebuild. I'm assuming they would be recruiting from the warzones where they fight. They could gather war orphans; the most promising among them will become battle brothers, while the others become chapter serfs, crew members of the ships, etc. The treatment of chapter serfs varies from chapter to chapter, but some of them are really well treated (compared to most citizens of the Imperium).
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