Roger’s comment that
I’d ask how important those echoes in the waking world are going to be, but if there’s either significant crossover or significant stakes in the dream world then this could work well for me
got me thinking about what kind of things could happen in my proposed campaign Gate of Horn. Here are some ideas:
Most basically, people who meet in dreams can find each other in the waking world.
Dreams that come through the Gate of Horn can reveal the future, or distant events, or hidden truths.
Dreamers can choose to remember things that happen in dreams and act on them in the waking world.
Dreamers can change the landscape or the inhabitants of the dream world, and by doing so change what other people dream.
Dreamers can be taken over by dreams and remain controlled by dreams after they wake up, perhaps as schizophrenics or somnambulists.
Perhaps instead of a sinister Necromancer there is a sinister Oneiromancer who has to be defeated.