Russo's confirmed that Death isn't in the movie and the balance thing is the main motivation for Thanos. Pretty sure Joss was going for it with the tease in Avengers and an interview where he gushes about Death a bit, but he [claims] he couldn't find the 'right story' (same Batgirl excuse).
I once speculated that Hela could take Death's place in the MCU for Phase 2. I still think that could have worked but the build-up to this is obviously better.
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