What do I need if I want to make a 3D game without a complete game engine?

I've been down that path a few times, and from a learning experience point of view I think it's a great idea. However, I would discourage you from aiming to create a game engine without knowing what kind of game you're making. Game engines only exist to support making games. All of the features you need to implement in the game engine becomes a lot simpler if you know what kind of game you are going to make. I spent about 2 years making a "Frankenstein game engine" as you call it from various other libraries. Along the way I kind of forgot what kind of game I was making and wasted a lot of time implementing features that I never used. At best, all it could do was some cool tech demos. Then one day I quit the project and refocused on making much smaller 2D games instead of 3D games and everything has looked better ever since. I still like to cobble together my own game engines from libraries but I do it only with the intent to get features working in the actual games I'm making. So in short, I don't think you are going to be able to answer this question until you ask yourself some others. What kind of game do you want to make? What platforms do you want to support? (e.g. Windows, Android, iOS) What programming languages are you most familiar with? And so on.. answer those questions and the answer to this one will be more obvious.

I honestly prefer to make games using the most I can from the tools I have. However it DOES have the inconvenienve that sometimes you can't develop that GREAT idea you have but that just can't be done with the scripting API your high-level engine gives you. So, you first have to setup your mind, do you want to make a game or a game engine?. This is because if you want to make a game but you can't find that functionality you got to have, doesn't mean you have to make a new complete game-engine. In that case, you just had to twitch a little bit the guts in your favorite engine (btw, you'd need an open-source engine) and use the other things at your comfort. But, if you'd like to build a new game engine, then I would suggest you check 's answer, but bear in mind, you won't be making a game you will be making a game engine and that's... well.... a pain (in my opinion). I used OGRE and CEGUI around 6 or 7 years ago and it is great for learning (actually I would highly recommend it for learning how a game works on the inside without having to build an actual engine), but I haven't use it in a long time because now there are other great engines in the market like Unity3D and UDK that lets you focus on your game and not in its guts.

Well if you finish your game, and your code is modular, reusable, and object oriented, then ultimately youll have a separate component for rendering, gameplay, File IO, controllers, sound, networking etc. That is essentially a game engine. The flexibility and feature set will be fewer than a fullblown commercial engine but it'll be more suited for and lightweight for your game. You can write all modules on your own and have more control on your code , which is great if you want to implement a very different game that current engines do not support.But in general, there are game engines out there for most kind of games already, so its a bit of wheel reinventing. The newer engines are very very easy to use if you're familiar with game-development jargon. That being said, the amount you'll learn by implementing your own is obviously much more than you can by reading about engines. If thats what you're looking for, building a small 2D game might be a great start. Ogre3D is already a full fledged rendering engine (IMO the crux of a game engine). If you want to learn more, you can start by writing your own wrapper around OpenGL. Thats the fun part of an engine. You can also write your own networking layer if it's a LAN based game. If you go about integrating libraries, you'll only get good at integrating libraries.