Interesting idea but since Dark Souls is based around being a singular undead badass and actually punishes you (not severely) for hitting those summon signs I feel like purpsefully throwing in content built for 4-6 players is counter-intuitive to what the gameplay was meant to be.
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How do summon signs punish you?
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Each summon gives a boss a bit of a difficulty hike and ups the chance of invasions. Again, not severely but the mechanic is still there
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By Buffing everything