The wish to use other game engines than the BGE is pretty old.
Non-Integrated development Environment
The “offical” workflow is that you
- use Blender to create your 3D content.
- export to a usable format
- import into the target game structure
- edit your game in the game engine’s tool set
This means Blender is part of the development environment, but it is not an integrated environment. Means: You have different independent applications.
Nearly all game engines support this workflow, even the BGE.
Fully Integrated Development Environment
Regarding the BGE Blender acts as IDE (Integrated Development Environment). You do not need to leave Blender to perform the above 4 steps. Due to the integration step 2 and 3 are automated and hidden to the developer.
The price to pay is … there is a single game engine that is supported this way (and with 2.8 there is no game engine anymore). Due to the architecture since 2.79 the BGE has side effects on Blender and vice versa. That makes the maintance and development of both Blender and the BGE harder.
There is a third approach in the middle of fully-integrated and non-integrated
Semi-Integrated Development Environment
This approach still uses Blender as IDE, but it does not bundle it with the Game Engine. Typically Blender gets several addons to be enhanced by editors to create assets of the target game engine. The addons can automate the steps 2 and 3 too. So it is pretty similar to the full-integrated development environment.
The big difference is that an instance of the game is always started outside of Blender. You can have a button to start the game, but it runs seperatly.
I do not really see that as a drawback. Even a BGE game runs more or less isolated (Blender freezes while the game is running). The options to debug a game within Blender is really limited anyway. Additionally you can be sure that the game will not behave differently when started from Blender and when started as standalone application (big problem on the BGE). The drawback is that the game is not running in a Blender window. I do not know if that is really important.
I know of three game engines that support this way:
- Armory
- Blend2Web
- BGE (yes, you can run it that way too)
When Blender improves the ability to customize the different editors (such as the node editor) it can help to let Blender serve as IDE on more game engines. With 2.5 there was a large step into this direction.
It might even be possible to create a BGE logic editor with 2.8+. Then you can develop a BGE game with Blender as semi-integrated development environment. The engine is still there (blenderplayer).