For HOMM II, there is a comparable project at https://sourceforge.net/projects/fheroes2. Source: about 1 year ago

Bethesda will likely never open source Skyrim, the game engine would need to be recreated from scratch. This has actually been done for Morrowind with the OpenMW project so it is quite possible it could be done for Skyrim as well. That said, Skyrim is a bit more complex than Morrowind in terms of what goes on under the hood so it would be a lot of effort by dedicated people which would take many years to... Source: 6 months ago

You want to use OpenMW - https://openmw.org/. Source: 6 months ago

Morrowind was the first TES game I played so I am biased towards it but the main appeal for me was the unique world and lore the guild quests feel more realistic for example the first couple Mages Guild quests are finding local mushrooms and flowers, gameplay is a bit outdated the combat is based on hit percentages based on you and the enemies skill and stats and fatigue level, the main thing I recommend is using... Source: 12 months ago

For those that have a liking to the blog style project updates some other projects do, the main website is a good feed https://openmw.org/en/. - Source: Hacker News / about 1 year ago

Morrowind - don't bother playing this without the Morrowind codepatch + Patch for purist OR you go with OpenMW. Source: about 1 year ago