Put a couple of hanger rods in the closet and you now have a 4th bedroom.

Its not officially a 3rd bedroom because there's no closet or door but it can easily be considered a 3rd bedroom.That model is the same size as mine, but I figure it can rent for more because its a quasi 3 bedroom vs my 2 bedroom.

If you are building to sell it, you can get a premium since it is Class A at that point, no capex needed, but you won't cash flow much unless you have closet to 50% down. 

I've seen "kitchens" that would normally be a closet.

The closets in the bedrooms have sliding doors and the bases are nailed directly into the concrete slab.

Excellent idea to drop one down on the floor in the laundry closet/ area to notify both the tenant and yourself that there is something that needs to be addressed.

My question is should we just put a master bedroom in the basement with a walkin closet and lots of storage, or should we also spend the money on another full bath?

If you want to go electric, get a few ductless systems or portable units that operate in heat pump mode and you will get 2-3x better efficiency.  2 ton ductless systems are about 900 or so on ebay and 1 ton portable units are about 200-300 online.

@Bethany TaylorIn my area, the builders add a lot of "extra" rooms without adding closets or doors.

Homes in the 30's and 40's had lots of charm but not open floorplans or walk-in closets.