I think having a large field in the next presidential primaries is good and in fact crucial.

Biden, Harris, Booker, Sanders, Warren, Beto and others all should run. Preferrably there should be at least ten serious candidates. No candidate should drop out of the race before, at a minimum, the first fifteen states have voted, and candidates who picked up a substantial amount of delegates in those should be encouraged to stay in the race until the very end, despite being "mathematically eliminated" at some point. These candidates can then encourage their delegates to vote for another candidate of their choosing at the convention and thus throw their weight behind whoever they find reflects their own platform most closely.

No candidate will likely end up with an absolute majority this way, and actually getting nominated will require some coalition building..

I think the perceived lack of options was the biggest driver of the dynamics of the 2016 primaries.