I think the growing, collectivized voices of the left are more interesting than a top-down notion of influential thinkers, but there are a ton of thoughtful, vigorous analysts too.

Nathan Robinson, editor of Current Affairs, is devastatingly clear and systematic writer, who's pretty much woodshedded Shapiro and Peterson's sloppy thought. If you want foreign policy stuff, Fellow Traveler's Blog is an independent source for discussion of left foreign policy. If Russia and Natsec stuff occupies you, then Marcy Wheeler's blog is perfect. Tressie McMillan Cottam and Eve Ewing are go-to's for education policy.

If you want an example of active and not reactive leftist ideas and policy, you should take a look at People's Policy Project. It's a response to the overwhelming center/right orientation of DC think tanks. It's funded by patreon, not by some creepy billionaire, and churns out thoughtful policy analysis on voting rights, medicare for all, public ownership of industry, social housing, etc.

https://www.peoplespolicyproject.org/