How much time do you think it would take to get a simple game environment with one of those engines, like a basic isometric view and a little guy the player can move? I've been tempted a few times to mess a little bit with that and try a few AI algorithms, but I'm afraid this could take way too much time.
It depends on the implementation.
If you do a 3D game - an isometric perspective is as simple as changing the camera angle.
If you want a "2.5D" sprite-based isometric game (Like Underrail, for instance) - you're probably better off creating a tile-based engine, which is a little more tricky - but there are a zillion tutorials and downloadable world-builders for that purpose in the asset store.
For animating characters, Unity comes with a built-in Avatar - with a rig that's easy to animate and with full standard controls, meaning you can more or less drag and drop the model into the game and use standard WSAD controls for movement during run-time.
All you'd need is a flat plane with a rigid body component as the ground - and it would work.
So, the basic functionality you're talking about is simple - and I could make a simplistic isometric game with a character moving around - jumping over obstacles etc. in a couple of hours probably. It wouldn't be very exciting, though.
Unity also has very easy-to-use tools for terrain generation and stuff like path-finding, which I've used in one of my projects. I have a Diablo-type game that I've made - mostly based on tutorials, free assets and stolen pieces of code - but there's basic fighting, spell-casting, looting, progression, primitive inventory management and so forth. There's even sound effects and such.
For custom characters, you can literally download character models and whatever animations you want - and combine them, if they both support the right formats. So, you could download a fairy tale princess - and then download military style tactical animations, including crouching and prone positions - and apply them to the princess.
It's more or less that simple.
Obviously, it takes time to learn the Unity bespoke methods and syntax - which is based around Game Objects and Components.
You have to learn how to reference each individual component and how to manipulate their properties - but it's really not very hard.