Usually it just means a second means of egress, a heating/cooling source, a minimum ceiling height, and a closet.

The strongest trend will probably remain the migration from north to south (FL, TX, NC), out of CA and IL and also a little more away from heat, water shortage, wild fire smoke and storms.Real estate markets move so slow, that short term is realitvley easy to predict based on leading indicators.

I ended up putting it around back, around the large cable going to the heat pump unit on the ground. 

Too many headaches for cash flow on paper (which is actually negative in reality) : stolen AC unit (thieves climbed a 7 ft fence), attempted break in (alarm scared intruder away), repair issues, winter weather (make sure someone turns up the heat on a vacant property).

I would think that if the inside of your house is heated you shouldn't have any problems, as that is where the batteries and everything of importance is located. 

Private, heated pool.  

You have to treat your tenants like guests and provide them an excellent experience  2) competition is heating up so you have to bring your A game.

Folks tend to not do anything when the heat is down low...but when that Sheriff serves them notice of the auction or eviction...it's a whole other story!

Also features that are desirable in duplex would be a separate heating system which allows the duplex part to adjust its heating and air conditioning to whatever the renter wants.

Plus its a non stop location for tourism Summer Camping, White Water Rafting, Rock climbing, 4 Wheeler/ATV, Hunting, Fishing.Winter - Ski, Snowboard, Tube, Ice Fishing, Hunting, Winter Breaks away from the Heat etc.