Maribel will eventually get in over her head with this dream-questing business (possibly involving some sketchy books of forbidden magic) and get sucked into the Gap, becoming Yukari.
Personal headcanon/incipient fanfic that will likely never be written:
After a long search, Maribel and Renko find a spell they hope will let them physically cross over into the world of their dreams. When they realize it requires a human sacrifice, they reluctantly abandon the plan and start avoiding each other (because awkward).
Months later, Renko contacts Maribel, saying that she needs to see her urgently. Maribel obliges, and Renko leads her inside, where she claims she's found a way to make the spell work. The room is laid out with signs and candles and such; Maribel is increasingly concerned. Before Renko can make her move, Maribel spots the knife she's concealing, and the two grapple. Knocked down and cornered, Maribel desperately grabs an old umbrella that was knocked over in the struggle. Renko trips over other debris in her next lunge and ends up with the spiked end of the umbrella piercing her neck.
As Renko stumbles back, sputtering and bleeding, her blood spills over the signs on the floor, and the room begins to shake. A portal opens, dragging the hapless Maribel into another plane of existence, umbrella and all.