I mean the niche best known for Bethesda's The Elder Scrolls III, IV and V.
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(Fantasy) The setting is a fictional world with fantastical elements, and a comparable level of societal progress as the 1600s or earlier.
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(Character Creation) You create your own role: character and class. It is a permanent character.
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(Action) You have direct control over your characters actions in real time.
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(Open World / Sandbox) You aren't forced to do the main story and can roam around the whole map finding items and doing side quests.
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(Single player) Not having to accommodate for multiple players, your choices can make a lasting impact on the world.
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(Alive world) There are events that can happen by chance, like meeting people on the road.
Less importantly, they are first person. This connects you more to the character, but downside is you don't see how cool armor you are wearing. First person combat might also keep it from reaching high action potential, although games like Mount&Blade and Kingdom Come Deliverance features good First Person fencing.
What are the competitors in this genre of single player, open world, fantasy, RPG?
Baldur's Gate is not an action game.
The Witcher forces you into a premade character and thus I don't consider it Free Role Playing.
Kingdom Come Deliverance is not fantasy and you are a set character.
Elden Ring has few NPC interactions, choices and well executed quests. The world is heavily hostile. I don't see it fulfilling the niche quite, but it fills my craving per now.
With Oblivion and Skyrim being real hits, why aren't there more competitors in the Single Player, Open World, Fantasy, RPG niche?
you should play the enderal saga , it's the closest to what you are looking for. It's a collection of total conversion mods for the tes games. Enderal is widely known since it relseaded for Skyrim but there's also Nehrim for oblivokn and arkwend and myar aranath for Morrowind.
8 personally love Nehrim so much I have over 300 hours played on gog and haven't even finished the main story yet