A) This isn't Eve online, you won't have massive ships even close to the size of the Infinity.
B) You won't fight other clans for areas to control. You're all on the same side, you're all guardians protecting earth.
C) There might be dogfights, Halo: Reach kind of hints at it.
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My reply to your point (b).... That doesn't mean all the humans are the same. they probably have a different point of view of how the traveler ended up, where it is today, and that will cause friction within humanity.
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Not when there is a greater threat to humanity than itself. Like in Halo, there was friction between the UNSC and insurrectionists, when the Covenant came along, that didn't matter anymore. You have 1 place to call home, that is the 'last city'. They'll have competitive multi-player, and that's called 'faction wars', but we don't know how that will fit into cannon, or if it will at all.