Do not attempt to create an engine if you have no experience making games at all beforehand.
I’ve been creating my own game engine for, funnily, also about 10 years by now. Since about 3 years I’m at a place where I’m seriously having better workflows that I could achieve with Unity, but since you mentioned that you don’t mind it taking 10 years, hey, there’s a chance.
But you need to have an idea about how other engines work, and games in general, before you being working on your own. For example, alongside my engine I’m developing an 2d action-rpg with an expected 40h of story-content. I started this project in Rpg-Maker XP, and many of the design-choices are inspired by how the rpg-maker is actually really terrible in many regards. So I aim to produce workflows that allow me to produce similar content, but better, larger, and with lot less overhead. How’d I know any of this if I didn’t previously try? I wouldn’t.
Similarily, working in other game-engines that are not quite as limiting as the rpg-maker can be a different source of inspiration. Even if I hate unity with a passion there are certain aspects that are worth mimiking. My visual scripting-language is entirely based around the concept of blueprints, again with my own spin and improvements, but not possible îf I hadn’t had some experience in Unreal.
So definitely get some experience with games to even be able to know what a game-engine requires, rougly. For reference, I had about 4 years rpg-maker experience when I started my own engine, and I’ve started getting experience with Unreal about 2 years after I began. Unity was even later in my job, but I definitely wish I’d started with both sooner, would have guided me in a better direction from the start.
