Someone already made a massive MMO in StarCraft, so it's not much of a surprise that something like this is possible too.
It's called Transformers: Open World. According to the ModDB listing, it's a third-person open-world RPG mod for StarCraft 2: Legacy of the Void that will allow users to drive around, beat up bosses, transform and upgrade.
"While the game is currently truly just in Pre-Alpha, what I want to achieve with 'Transformers: Open World' is a visually stunning, atmospheric RPG experience with mechanically advanced gameplay, a huge open world as well as a pretty large amount of content," creator Crainy wrote.
In a posting on Teamliquid, the creator added that players will be able to combat various Decepticons. The mod is currently in pre-alpha, but basic melee attacks, projectiles, interrupts, dodging and charge attacks.
It's an ambitious project, although the engine and map editor is certainly more than capable of supporting efforts like this. StarCraft Universe was an MMO mod that became so large it broke out into its own Kickstarter campaign. The beta for that was shown off in November last year, although the website still lists the game as being in an open alpha phase.