When you finish the campaign(s), check out the custom commanders in Cooperative play. Han and Horner are my favorite! 
I absolutely love Coops.
Every one of the leaders has something cool about them.
Some are amazing on early offense like the above Han/Horner, or Zagara, some on the other had are late offense team but early on Defense they are insanely hard for the AI to break into like Karax.
Coops are also a nice combo of Multiplayer PVP sort of thing & v/s the AI in the campaign.
Its you & another live person v/s the AI, hence "cooperative" and the units used are not quite the same as either the MP "standard" units or the various "campaign" units.
Each leader has a tailored selection of units from their faction & often w/ only some of the "evolutions" done to them.
It's hands down my favorite play type. Love the campaigns but PVP is meh to me, if you get too much lag then it doesn't matter about skill.
I have most of them unlocked, I don't think I did the last 2, Egon or Mensk as I got into another game & stopped playing SC so much.