Ideas dump. You may or may not get sued if you steal one of those...
Organ Memory
Genre: Survival horror/Puzzle solving
First person perspective
Based on the pseudosceintific theory that memories can be stored not only in brain, but also in all individual cells of one's body.
The story is simple - someone dies and several people receive organ transplants from that person's body. You are one of them. A long while after you're out of hospital, your health gets much worse. Further examination reveals that state of all your organs gets worse - except for the transplanted one, which grows stronger. In some time you receive visions from organ donor's life, and strange paranormal occurences happen all around you. You find out that other patients after transplantations are in the same situation. You must find a way to get out of this shit, and (not necessarily) help others.
TLDR: Organ donor is a douche and returns as a ghost to kill you and get it's organs back!
Necromaster
Genre: Strategy
Isometric
Evil vs Evil scenario. Deep in enormous caverns under some accursed desert lie ruins of a great ancient civilisation. After ages of lying dormant you awaken - one of the Death Lords that brought aforementioned civ to it's ruins. Your goal - breach to the surface. But to do this, you'll have to get an undead army, and you're in a wonderful place to get it, as that place is an enormous, ancient graveyard! But alongside you, other death lords awaken and each and every one of them wants to rule both undeground and topside world. War of the undead starts!
Player would have unlimited power, but would be limited by resources - bones needed to animate undead. Excavating them from ruind and graves would be necessary. There'll be several starting units which would split into varieties based on one stat, like mobility, offense, defense. There'll be also neutral or outright enemy units - like ghouls that'd attack both you and your enemies.
Undead Hunt
Genre: Strategy/Survival/Hack and slash
Isometric
Humanity turned away from gods after plague outbreaks and great war - gods turn away from humanity and open gates of Underworld, letting countless undead and other monstrosities like demons or dark spirits into the world.
In singleplayer campaign, player would control a group of undead hunters that have to clear off some area, survive, find out what caused the cataclysm and stop it.
In multiplayer, player would control only one character in given scenario and would have to cooperate with others to complete objectives or survive.
Edit: Forgot to mention this one
Nocturne: Black Heart of Jeramesh
Genre: Survival horror/puzzle solving
Third Person Perspective Shooter
Remember the old game Nocturne made by Terminal Reality, where you played as an agent of secret government organisation fighting monsters and paranormal? It ended with a horrible case of a cliffhanger... But this idea is not to continue story after that point. It's rather like one of chapter's of the game - seemingly unconnected story, just another case. This time, Stranger would be sent to investigate mysterious abductions and paranormal phenomena in the city of Jeramesh.
Who breaks most of the furniture, wins.
This one was taken from a Blockland gamemode:
Santa roams around the town, throwing pieces of coal at 'misbehaving' children.
The misbehaving children (players) have to pick up presents and throw them into an incinerator or something.
Lol.
I might steal that isometric evil versus evil idea >:}
A game where you're flying in first person perspective around some place, scaring anyone daring to enter it would be more interesting. Especially with Thief-like well implemented AI that reacts to noises and environment changes (The DarkMod has such AI on id Tech 4 engine, and I'm going to port it to SE2 and mess with it anyway - it won't be too hard since they share base Tech 4 AI). You could grab and throw objects all around - try to scare or maybe even kill people. DarkMod has "grabber" code as well, it even was in Doom 3 RoE, though in different form - Tech 4 has in-built physics engine as well.
I think that it won't be hard to do on Tech 4 at all - you give player ability to fly via simple edits and build levels out of monsterclips (making outern-most walls normal brushes or models), so you could still pass through walls and floor but not escape the place. Port AI and grabber code from Darkmod, then create assets. You could make some more advanced stuff like ragdolls to control mannequins or creepy dolls with in-built tools and modelling software (model in Blender, AF Editor for ragdolls, can't remember where to make additional collision meshes), then add script allowing you to better pose models.
Or you could do the same on UE4, but you'll have to blueprint/code whole game logic while you have all of it GPL on Tech 4, and it's forks are decent enough for developing something more advanced than HL mod.
I think that isometric evil vs evil is the best one there. You can try, if it comes out great then I'd play the hell out of it.