-Symbionts. Perfect for a Mythic Menagerie. Just their stats for combat/encounter, how to join with them and more importantly their gp value cause they are items down the end.
-Grafts/implants. Plant, aberration, undead, fiendish, what ever. They were cool and their gp value was useful.
-Artifact spells. Like in Secrets of Xen'drik. Unique super spells. What was missing was the fluff and originality. Places where each of these spells can be found, their history, and use beyond combat.
-Magical locations. Locations as treasure, like in 3.5's DMG 2. Maybe call them eldritch sites? But they were a neat idea that could have been expended a pund.
-Battle suits. Not necessareely mechas that require their own rules, just suits that give certain abilities. An armor forr fighters, an organic three for druids, shadow stuff for rogues, a ice suit for wizards, etc.
Thanks.