There are things the government could do to incentivize corporate models that are pro-worker but not necessarily unionized. For example...
Employee Stock Ownership Plans (ESOPs) - Government could encourage more of these structures where every employee has skin in the game, or help establish new models
B-Corp Certification - This is an independent certification that evaluates (among other things) governance models, transparency and fairness to employees; Government could encourage these through tax incentives (I don't think there are any today) or other means.
Policies around corporate benefits - A major argument against employee benefit plans (especially healthcare) is that employees are held captive so there is less mobility of the workforce and less incentive to take risk with new initiatives. You could take this to the extreme and remove employers from the equation with universal healthcare, but that's not the only option. The ACA helps mitigate this.
Personally, I think income inequality between executives and front-line employees is problematic. I would be curious about government incentives to tie executive compensation to employee comp.
Also, these ideas and the general push for more pro-worker policies aren't limited to the liberal/Democratic perspective. Oren Cass is one right-wing voice who is advocating for Republicans to adopt more policies that consider all workers and stakeholders other than just shareholders.