Since then I've purchased my primary residence, completely rehabbed and doubled it's square footage, and I'll be renting it in May(Waiting until may to reach my 2 in 5yr).

Assuming that the place doesn't look bad, and that there isn't something obvious preventing it being rented like being in an absolute warzone, then all I can think of the square footage or layout.

Can you have room mates, rent out the garage for storage, repurpose extra square footage and create another bedroom, etc.?

Check CC&Rs and other local ordinances before you purchase the plot of land because many areas have minimum square footage requirements for new construction.Because it's on a fixed foundation, you will also have to abide by the International Residential Code. 

Some examples; can you add sq footage or adding an addition since the location is so great, can a 3/1 be turned into a 3/2, if the house is bigger can I make a 4th bedroom or 3rd bathroom etc.. if the return is there to do these type of things, you can really make the numbers work for the upfront costs.

These price adjustments can be for anything including condition, footage, unit count, bathrooms, garages, time sold (values have fallen ~10% in many San Diego markets since May), etc.  

The house has a lot of potential to add square footage and create more value, and we got under contract.

Depending on how eager you are to get started, you might consider waiting a bit as ADU/DADU rules are probably about to change in the City of Seattle, with requirements getting considerably easier such as no dedicated parking space required, no owner occupancy requirement, and larger square footage allowed. 

Good idea in combining rooms, saw a property converted from a school to multis and that is what they did to expand the square footage per unit.

Say we decide to move on this deal and decide to reconfigure the floor plan, adding a bedroom, and changing the flow of the house while keeping the same square footage.