TBH if you wanted access to Harshville's source code, you'd be better off just making a harshville-like game from scratch and make it open source. It'd be hard to keep it open source though. If you could find coders interested in contributing, it'd be difficult to jump neck deep into another project and quite a bit of effort to make additions with some semblance of cohesion; quite a bit of debugging and untangling with multiple coders entering and exiting the project.
Art is another animal entirely, as you'd want a consistent 'style' across all artwork for a cohesive experience. For 2d art, it effectively limits you to a single artist which sort of kills the potential for small contributions from an open community. 3d art is more reasonable, but there are other challenges (besides the still somewhat stylistic techniques for posing, rendering and lighting) with assets. Most free assets are copyrighted or unusable for the project (no NSFW features). Good assets cost money and are licensed to a single user (Daz, MMD, etc...), though if pirating assets in a completely free game isn't an issue you at least avoid the risk of DMCA. Open-sourcing other users' pc's as render farms is a huge possibility for one of the biggest bottlenecks with 3d art though. If you otherwise had copyright over your model and could release it for widespread use, it'd be relatively easy to 'crowd source' art, as 3d art is a much more manageable skill than 2d art.
One idea I've thought about for a while is using 3d toon shaded models to create 2d art (think dragonball fighter z, genshin impact, etc). These types of models however are simply not available with NSFW features as the technique is still so new and very few artists have the skill to do it. There are some 3d modelers taking commissions, but you can expect to pay $2000-3000 for a highly functional model. That model however could pay for itself ten-fold if community funded, and would create so much better art than the usual Daz game.