A good ideea for an expansion would be the abilityto start the story from different points.
The current warrior gets bonuses for human troops so it is forced a bit to use them or give up the bonuses.
You could instead start a main quest for a crystal of creation (or something similar) sought by all races.
You can play as each race and set the difficulty of the quests based on the distance from the starting homeworld.
For example if you play human the difficulty of the human quests is 10-50 and demon land quests is 50-95 but if you start from demon the demon land quests are 10-50 and the human quests are 50-95.
The main search can evolve on neutral lands so that the difficulty will be the same regardless on the starting world.
In this manner the demon warrior can have skills benefiting demons.
Same for mages: make demons more gifted for fire magic, undead for death/poison spells, dwarf mages could use ice spells and so on.
I could make this a 20 pages post but you should be playing not reading so I end it here
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