Keep in Mind that if you change the Engine and the Perspective, then you would pretty much have to design all Scenes three Times -- 2D with Adventure Perspective, 2D with isometric Perspective (typical for Strategy), and 3D for the First-Person-Shooter.
That would sound highly inefficient for the Developer, thrice the Work and rather unnecessarily so, because think about it: You could just as well use the isometric Perspective for all three Characters, and still have their different Skills, like in the Commandos Games (Strategy), Isometric Shooter's and Hack-'n'-Slays (Diablo), and isometric Adventures (Sanitarium).
I have to admit though, the Idea to see one Scene from Eye-Height, from above, and from First-Person-Perspective, sounds quite charming. But in that Case, you would be better off using a 3D-Engine (Unity, Unreal Engine, ... I personally love Blender Game Engine, but you'll hardly finish your Game with that one), so that you have to build your Scenery, Characters and Objects only once -- which would be much more efficient.
However, I have to admit that there is something annoying about it when, in Games like Commandos, different Characters have different Skills. Yes, it makes Sense that only one Guy in the Group really knows how to handle all Sorts of Explosives... but in the first Game only one Guy in the Group could drive? That was a little ridiculous.
Now imagine only one in your Game's Group was able to hide behind Walls, sneak around, hide, and the Explorer and Gunner cannot sneak and hide at all. And both the Strategist and the Gunner cannot do "Adventure"-Stuff, i.e. examine Objects, use Objects, and talk to People. Pretty strange, except if you have a good Reason for it, like the other two cannot speak the Game-World's Language and therefore only one can have Dialogues, or Guns are prohibited and nobody knows how to use Guns, except the Gunner who comes from a different Culture where they have Guns. And maybe the Gunner and the Explorer cannot sneak and hide because their heavy Armor / Explorer-Backpack are too sturdy and too loud. But why the Strategy and the Gunner wouldn't be able to examine and use Objects, I can hardly imagine. (Though "examine" could in this Case mean "analyzing with special Knowledge which the others don't have".)
Anyway I am just rambling. You worked on the Idea for 8 Years, so I am sure you have a good Idea of what you want and how it works.