G'day I've attempted this a few times. Especially with my games 'Bane of Oakenshade'. I've had a few misses along the way on some retro unity projects that have that open world style. Check out 'Citadel Unchained' . It's a massive open world but I had to sacrifice some features due to some coding bugs but you get the idea. I'll add my games also run on older hardware aswell. Oakenshade can run on win98 I think and Citadel Unchained can run on win 2000/XP.
I've helped integrate them into modern systems aswell but basically mainly inspired by Daggerfall/Kings Field and Morrowind. As well as countess other Late 90s early 00s RPGs
There's a huge collection of Kings field fan games to download free online if you like that first person open world style.
Also check my 3rd and 1st person game collections on my page. There's heaps of games from a huge variety of Devs. I think I've got almost 100 games featured in the 1st person collection.
I do always recommend an Awesome yet incomplete game called 'Moratheia'. If you can track down a copy it's well worth your time. It's a damn shame the developer abandoned it and never finished.