In this case you must have more detailed graphics for world and character.

It depends. If you choose an art style, which depends heavily on details (photorealistic direction), then a diablo like camera would avoid close up views and therefor you would need less details. On the other hand, many stylized games (diablo III itself is stylized) demonstrates, that a FPS with low details works too (eg TF2). Comparing TeamFortress 2 with Path of Exile, I would say, that it needs much more work to create Path of Exile art than TF2 art.

At least it is a good idea to reduce the workload, nevertheless, you need to limit your world extremly to handle the asset creation, regardless of what camera style you choose. A good example is grimrock , which was made by 4 people, has a clear vision (dungeon master clone) and a very limited scope, still it is FP and has amazing photorealistic art.

Storywise, I think that whether the story revolves around characters or around events could help you decide which camera approach is better. The further away you are from the characters, the less you see things like facial expressions or body language. Of course, as others have stated, this has no relevance if you (the developer) control the camera during cutscenes.

I happen to favor isometric views because the game seems more independent from my own actions than from a FP perspective. Curiously I feel more immersed when my mind can be fooled that the little people are moving of their own accord, merely guided by my will.

From my experience both playing games and developing my own, this is fairly typical response. For example I always prefer first person perspective as it is both more immersive for me and easier to control. However, some else playing the same game might (as indicated by DavitosanX above) find a third person or isometric view better. Could be because of experience with cameras in other games, whatever.

As a result my advice is to be flexible and probably give the player the choice. With the ability to switch from first to third person perspective this adds a bit of work, but with care usually not too much - the camera classes I've implemented in the past are pretty compact with the only difficulties coming when I wanted to implement six degree of freedom.

Looking at your specific case the difference between third person perspective and diablo style might be very small (as small as having a third person perspective camera fixed to a predefined angle and distance from the character or scene). Even switching from full 3D to an isometric display can be pretty simple (switching from a dedicated isometric engine to 3D can be trickier!)

As suggested try all options and see what works for the team - it may be that the prototyping may highlight one option that really gives the game the right feel - it just feels right and everyone agrees on the approach going forwards. And if not then you have several options up and running with no need to select one or the other - this becomes the player's choice.