Overview
According to many people you shouldn’t be able to do this, but after continuing the main quest for a bit you will be able to create your own faction.
General
After you ask the nobles about Nerethes’s Folly (first main quest) you get some quests to (spoiler alert) Assemble the Dragon Banner which allows you to create your own faction afterwards.
Example
Prerequisites
1. Be independent, don’t be vassal to another Kingdom
2. have at least one IMPERIAL Settlement (if you want to restore the Empire) or at least one NON-IMPERIAL Settlement (if you want to destroy the Empire)
3. your Clan must be Rank 3
4. you need to have 100 troops in your party
5. reach a certain point in the main questline (see General section)
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