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Just do a spiritual successor so you could actually commit to it. Black Mesa started development around 2004 and is only now finished. You are fucking insane if you think this game is going to be finished, no matter how hard you'd try it takes levels of hard-core Half-Life autism to finish an ambitious fan project. And even that didn't end up being free towards the end and would had collapsed without Valve's blessing to have them monetize it.
The Bionicle project holds promise unlike many others, but I'm realist enough to know that even that is likely an empty one, even if what looks like to be the unreal engine is far easier to use and not all assets have to be made, we are still talking about assets and resources needed more than a standard indie game, probably closer to AA territory.