I recommend you throw away Manual of the Planes altogether, and likewise anything from Moorcock, and think of your own ideas, or borrow from history or other inspirations.
Think "Twilight Zone." Think "Tir Na Nog." Think "Mirror, Mirror." If you have an enemy to fight, such as marauders or zombie soldiers or whatever, have the player characters discover a full-length mirror which plunges them into another dimension where the enemy has won, and the streets are overrun with them with the last remaining humans putting up a spirited last stand.
In that alternate universe, perhaps people whom the characters once lost could be alive - though they may never have heard of the player characters and be distrustful of them, or even downright malign towards them if they come from a "mirror" universe where everybody's personalities are the inverse of their own.
Alternate dimensions are the perfect "there but for the grace of God" lesson - if the characters fail to be diligent in the pursuit of their quests, however hopeless they seem, this scenario of abject failure awaits them.
If you have access to alternate dimensions in your games, you will need to invent sorcery spells and items permitting transportation to those planes: magic rings, mirrors, doorways or strange cabinets of wood panels, painted blue, which are actually bigger on the inside ...
As for planes of otherworldly entities, afterlife planes, a Spirit Plane, the Astral Plane ... Perhaps you only need one such Plane, taking on many different forms - Heavens for some, Hells for others, an animistic Happy Hunting Ground for yet others - and all the spirit-based magics of Common Magic, Divine Magic, Sorcery and Spirit Magic all access the same intangible place. The "demons" of summoners are the same as the "spirits" of shamans, given the vestments of flesh formed from their summoners' respective fears, desires and expectations.