I've had this idea for quite long and I finally wanted to share it. There are stuff, which I haven't thought enough to share them but I think the basic idea of the game can be found. I'll add the rest later. So, this is pretty much the very basics of the idea. Not even a proper story added in, hmmph.
Basically, you control a Necromancer, who has been sent to 1990's Tokyo. I haven't decided what kind of story it would have but it's most likely either to hunt down a heretic, who is trying to cause wars between all the creatures. Or the player itself is that "heretic".
Since you're a Necromancer, you can obviously use dead creatures to help you. Either by assisting in battle or by using their tricks yourself. There are at least few different creatures, who you can use: Humans, mages, valkyries and necromancers. You can use them by collecting their souls, which is only possible if you inflict serious damage to them but don't kill them. Taking their souls obviously takes some time, so it wouldn't be the best choice to collect souls during middle of a large fight.
Humans are the weakest creatures and offer nothing unique but they are easiest to gather souls from. Mages allow you to use magics the mages themselves had. The magics are mostly offensive spells due to the fact that mages love to torture their victims. They obviously aren't the friendliest mages you know so collecting their magic is difficult. Valkyries can fly and are great in either long-range or close combat. Taking their souls allows you to fly and you can receive either weapons or close combat moves from them. They are difficult to take souls though due to their ability to fly. Taking souls from Necromancers gives you a lot more souls and get new soul based abilities. They are increadibly diffcult to defeat though.
There are multitiple ways you can use the souls. If you have only collected the souls, you can summon the creature back to either aid you in battle, control it or use it as a disguise. If you control a creature or use it as a disguise, you can outspeak or even command the people of the same race. Disguising yourself as a creature reduces the chance to do it, but that way you can use a fire-based area magic againts enemies surrounding you.
You can also stack the soul to yourslef, either by linking it or absorbing it. By linking you can use the creature's powers but you'll lose the soul, if you take enough damage or another Necromancer steals it. Absorbing the soul increases your stats for something specific (Fire resistance from fire mage for example) and you can start learning spells or abilities similiar to the creature. Absorb enough amount of them and you have a permanent spell/ability to use. You can still lose the absorbed soul though.
The health system is based on souls too. When linking or absorbing a soul, you always gain health increase. If you take enough amount of damage, a soul will disappear. This usually happens, if you other wise died from a hit. Linked souls are destroyed first, disabling the abilities of the souls. Losing absorbed souls doesn't disable any abilities you have received from absorbing the souls. Losing controlled creatures damages your health.
For example, the player takes few human souls, two mage souls and a valkyrie's soul. The player decides to link a mage, absorb another mage, link a valkyrie and a humans are controlled. The player then controls the humans, moves them to group of other humans and successfully manages to speak the group away. However, enemies appear and he/she need to fight. The controlled humans are killed and you take quite a lot of damage since they were most of your souls. Player uses the linked mage's magic to kill few of the enemies. Unfortunely an enemy manages to backstab him/her and lands few very dangerous hits. The player sacrifice linked mage's soul to survive. After that the player takes more beating and valkyrie's soul is gone. And soon after that the absorbed mage's soul is gone, taking away the stats bonuses it gave. The player can still use his/her own magic though. The battle ends with absorbing a mage's soul to gain an extra spell.
The gameplay leans towards free-roam RPG, which is the direction went with my idea. In Tokyo you could help others to solve their conflicts (Other races' problems obviously only when you're disguised or controlling a creature). Harvesting too many souls can gather attention from other Necromancers, who will try to hunt you down, either in order to gain more power or due to the goverment of the Necromancers ordering the player to be hunted down, because of his/her high threath level. The races would be seen fighting their wars once in while and it would be even possible to drive them away at least for a while.