The problem with just using Blender to build your world is that a good world is more than just geometry and textures. It also includes scripted events, encounters, dynamic objects, etc... which are harder to handle. You could hack something using Blender. Maybe something like creating named Empty objects at event locations, and naming the Empty to be parsed by the engine to know what to place. It's hackish and inflexible and doing anything non-trivial that way would be a monster. In the long run, your time would have been much better spent on the level editor.

You could also just do that stuff outside Blender, hand-writing a bunch of scripts and placing things manually. It would take a lot of iteration, a lot of loading up in the game and playing for a few seconds, then reloading, etc... Still would be a lot of work.

You would have to just estimate how long it would take to do things by hand like that, vs how long it would take to build a proper editor and build the map there. That is the only way you'll know if you're actually saving any time by not building the editor. I suspect that if the map is of any non-trivial size, it would actually be much faster to build the editor and amortize the time spent on that as savings throughout the remainder of the process.

If you're reusing a lot of level elements, or need to add a lot of additional details (script hooks, trigger areas, etc.) then it's probably worth creating an editor. If elements are all (or mostly) unique and you don't need to add a lot of additional information added then you might consider saving the effort.

You also have the choice between the creation of a complete stand-alone editor, or simply adding some basic in-game tools -- which may be less effort than a stand-alone editor. You might also consider a standardised format for which you are able to use an existing editor.

I thought that level editor is better if you are going to make mostly unique stuff? Because you can build buildings any way you want with wall tiles etc?

Apologies if this wasn't clear. When I mentioned "reusing a lot of elements" I was specifically referring to "building block" type elements such as tiles. The alternative to this would be to create the complete asset (model, sprite, whatever) in an external package and import it as a single element rather than building it out of smaller reusable pieces.

Consider as an example the Baldur's Gate series of role-playing games: each building is unique, and they are not built out of the same tiled elements; each structure is a complete model. In this case, it might not be worth the effort to create a fully-fledged editor, as the bulk of the work is already done in whatever modelling package created the assets. An editor may however still be valuable for creating or adding extra data such as triggers for scripts, data for AI, etc.

So do you think I should ignore using unitys "map editor"

If you're using Unity, and you're not generating your levels procedurally, then you should probably be using the editor provided; the powerful and (relatively) easy-to-use editor is one of the main benefits to choosing a package such as Unity in the first place.

One valid reason for ignoring the provided editor and creating your own would be if you intend to distribute the editor with the game to your end users and allow players to create their own maps. Even then, until such time as you've created your own editor you might consider using the provided editor to get started with creating some initial test-content.

One thing though.. I just realised reading your comment about adding things ingame.. maybe only for DEVs accounts...

Absolutely, this is something plenty of games do. You might allow the features only for specific accounts (such as developers), in specific builds of the game and then remove them from the release, or you might make them difficult to access with obscure shortcuts -- "cheat codes" in many games are actually features that were originally included for developer use. Building in editing features -- rather than a stand-alone editor -- can be particularly useful if you just need to add a small amount of data rather than constructing detailed levels.

Do you have any links to standardised format of existing editors that I can check out?

You might consider any existing map format. As an example, you could use the format supported by Valve's Hammer Editor, or if you were using an older-styled game you might use the same format as Doom, Heretic, etc. and use an open-sourced editor such as this one. You might consider any tools and format you are legally able to use.

For a game like this... do you think first thing to work on for the game is the level editor?

Hard to say... you haven't really said a lot about the game, but an editor might be useful for a non pre-rendered game. Personally I'd start by implementing some of the game itself using simple hard-coded map-data.

Of course if you use Unity the question is moot -- the editor already exists and is ready to use -- in this case (or if you're using any other similar package) you should start creating the game.

I'd start by putting together a small area in which you could test new features (abilities, weapons, whatever) as they are added.