RPGs that are "true" open-world

[CryptRat]

The following games are mostly focused on party-based tactical combat except specified otherwise and you should be able to do most things in any order as long as you're strong enough.

These ones are fully open, including the ending parts accessible from the start :
Disciples of steel (I'm not 100% sure about this one but this is still one of my main recommendations)
Horizon's Gate (this one goes all-in the villain is just here go and fight him whenever you want)
Ringlorn Saga Gaiden too (more reminescent of old japanese action RPGs though)

These ones are quite open too but either the ending parts need to be unlocked by doing something when even not basically everything or I can't remember if they need to be :
Helherron (still possibly my first recommendation despite this)
Natuk (which might belong to the first list but I can't remember for sure)
It's among the most cool things about Aethra Chronicles.
Same with Voidpsire Tactics.
Death knights of krynn (this one is cool as you liberate cities in the order you want)
Lurking I: Immortui, Lurking II: A Madness (these ones you don't play for the combat but for the Ultima things)
Jewel of Arabia : Dreamers (you need to manage to make this one work on a Mac emulator, I think it's kind of worth it although not my favourite in the list anyway)
I think Legend of Faerghail kind of fits and it's one of the things I like about it although it's a buggy mess more than anything else
Bludgeons & Krakens (you control only a single character in this one)
Monochrome Kingdom
myrkvidr is advertised this way I think although I've only played a bit, it might be an interesting recommendation anyway, also more Ultima than combat
Artifact Adventure, especially if you choose the spaceship at the start but kind of if you don't too if I remember right, the world is basically shaped like in an early Dragon Quest
That's the main hook of Antharion, whether the game is worth it is more questionable.

Kind of open but good either way :
Gates of Skeldal (the biggest part of the game you can mostly do in any order unless you need more key items from one part to do another than I remember)
Sword & Sorcery : Underworld is quite less linear than your average Bard's Tale or early Wizardry as it's inspired by Might & Magic.

 

[mindx2]

Not sure if Might & Magic VI fits what the OP is looking for but you can go anywhere from the start. You'll get massacred of course but you can visit any map or dungeon if you run real fast.

 

[Iucounu]

All areas of the game must be accessible from the very start of the game.

If you interpret this literally, then you should exclude any game that has at least a room with a locked door with a key that you don't have in your inventory at the start of the game. I don't think this is a good criteria.

I don't mind being locked out as long as there's a plausible explanation, in fact gaining access is often the source of fun gameplay. What's bad is when games create arbitrary barriers.

A better criteria could be that, when you access/open new areas these are interconnected with old areas and old areas are still accessible and not secluded.

Are there no games where returning to previous areas becomes a new gameplay challenge? Stalker comes to mind, of course, but usually games condition players to constantly press forward and never look back, which seems a bit wasteful of game assets.

 

[KeighnMcDeath]

Does Starflight count because man oh man... you can go anywhere. And a lot of times you probably shouldn't. It is beyond open world; it is open WORLDS!

 

[Damned Registrations]

Man, Starflight was good stuff. Why can't we have more games like that?