I often look for inspiration in the 1996 video game "The Elder Scrolls II: Daggerfall." It was developed by only 27 people (excluding beta testers), which is a small amount when compared to modern teams. Daggerfall ran on XnGine, one of the first true 3D game engines. In terms of gameplay, there is a lot of influence from the roleplaying game Dungeons and Dragons and earlier first-person PC RPGs like Ultima: Underworld. Aesthetically speaking, the art direction draws clear inspiration from medieval fantasy art. The lead artist, Mark Jones, worked on many DnD themed games. As to what we can learn about the game's immersion, I'll provide examples of what players can do: buy and sell buildings, loan money from banks, barter with merchants, own boats and explore a region the size of the United Kingdom. If all this was possible in 1996, why can't we do it now? I admire this project because of the ambitious world and the hand-drawn art it has.
It's worth mentioning that a small community of modders ported the game over Unity 3D and Daggerfall has never looked better!