I posted a few things in the custom investigators section, but I forgot I have a few Ancient One concepts that people might enjoy.
Starting with Crom Cruach.
Inspired by the creature from the animated movie "The Secret of Kells" (and also of the Pagan deity of the same name from Celtic Mythology), Crom Cruach creates a board state that functions oddly, yet in a fun way. The most lethal thing that can happen against this Ancient One is a gate opening on the Woods, but sealing that location becomes especially prominent here as well. Hope you and your buddies enjoy.
Next, I have Bloody Marie, from a recent pioneer project in video games: "Skullgirls"
Originally, I had tried to create a very complex "wish-granting" function to this Ancient One, but ultimately it came off a little too similarly to The Lurker at the Threshold in terms of mechanics. Plus, I wanted there to be at least 1 other Ancient One that interacts with Mask monsters in our box of Arkham stuff. Though I didn't include much of a wish-granting mechanic within the normal boundaries of the Ancient One, all of the sinister plots revolve around an old version of the "wishes" system.
Finally, from the popular playing card game "Magic: the Gathering", here's Lorthos the Tidemaker:
Straight from the depths, Lorthos covers Arkham in a massive flood! This is an Ancient One designed to make a non-relevant monster type (Aquatic) suddenly VERY RELEVANT. Understandably, making every part of Arkham aquatic is a bit game-breaking at times, but the number of interactions is fairly limited since there aren't even a surplus of aquatic monsters in the monster cup most times (I personally play with everything in the monster cup, but I don't know what other people play with).
Down below is a download link with the Strange Eons files for all 3 previewed Ancient Ones and a full set of Sinister Plot cards to compliment them.