Not gonna lie, actually working on a Stalin dies TL right now. Trotsky dying during the revolution would be big, though, because he was one of the most influential figures in the Red Army and I haven't spent any time thinking on that yet. Depending on who he was replaced by, the Civil War might have ended rather differently. Still a Red Army victory almost definitely, but some countries that were or weren't under their thumb in OTL might switch over.
Purely spitballing, but seconding collective leadership. The Central Committee would remain incredibly strong and Leninism would be the unchallenged ideology of the country, but as soon as Lenin dies factions would emerge and it would be hard to keep them down because no one in the party seemed quite as ready as Stalin to oust political opponents. I would imagine that the transition to "socialism" would be a lot slower, because Trotsky was one of the biggest advocates for a rapid break from capitalism. Bukharin and Zinoviev would definitely be big players. Whether or not the regime would be stable for long is really up to the author, because the Great Purge being butterflied completely changes who is and isn't at the top of the party.