Some great ideas so far.
For me the big thing would be to start with a gooseneck. Once your used to pulling them they are more maneuverable and pull much nicer then a bumper pull especially when its windy or icy.
This is clearly becoming a theme in so many different replies..
I would want a built in RV charger converter and probably an inverter also that way if your on shore power, generator power, or running off batteries you always have 12 volts and 110 volts.
Clearly a must have.
A solid work bench which a gooseneck comes with built in and a nice storage area all in the front area.
Built in propane tank compartment or underneath mount is nice for the furnace.
If I go with a bumper pull, I need to see if I can get the twin tanks OFF of the hitch and under the trailer without compromising ground clearance.
Some bright thin light florescent lights for working I used 12 volt ones in my current trailer that can run off the battery or rv charger converter.
Lights, Lights, Lights.
Everyone wants MORE lights.
Wired with a couple 110 outlets and a few 12v receptacles also seems like my phone is always dying when were working on sleds.
Gotta have power.
A thought.
Wonder what it would take to have a little USB panel built so anyone could charge a Phone-iPad-iPod??
Loading lights in the back are nice to. I wired a diode and a switch into my two LED backup lights so I can turn them on when were loading and it doesn't back feed to the pickup either.
External lights front, back and side are on my list!