Best Value adds are:Updated kitchen, updated bathrooms, cosmetic remodels.Additional bathrooms - finished basement spaces, in unit washer dryer, central air, additions/alterations.All of those are useless though if your roof is 30 years old, your mechanicals are from pre 1950's, and your home is in dire need to be renovated "behind the walls"Otherwise the above will increase your value but you will be left with Lipstick on a Pig

Fire sprinklers, plumbing, sewer lines (to city main), electrical, kitchen, bathrooms, flooring, driveway with four cars parking spaces, and foundations.

New everything: fire sprinklers, plumbing, sewer lines (to city main), electrical, kitchen, bathrooms, flooring, driveway with four cars parking spaces (it did not have one, can you believe it?)

If neither is an option then you would either have to get a zoning change or duplex down to LEGALLY use the space. 

and seating so buyers can imagine how they might live in the space.

Find a place when a ton of space you won't use and rent it out. 

You cannot just raise rents 50% Not sure what your cash reserves are or if you are planning to 1031 or something along those lines but I would rather see you keep cash in your pocket and apply it to a rehab that involves addition or updating kitchen/bath so you can see sizable jump in ARV and rent increases.   

Listing contained no interior photos and a limited write up with most who viewed the listing focused on "two retail spaces and "9 (1) bedroom apartments"  At first look most would pass over this because they would see $195K/unit with 9 being 1 bedroom units. 

Definitely would stand out as a unique space.

For your example, making the basement fully operational as a livable space seems like a great value add.