Here are my suggestions:
Sand Blast and repaint whole trailer (light color to keep heat down)
In nose section remove that little door and replace with a window for light (if one on both sides do on both sides.)
Section off the front and put an area of tack / living quarters. Install cabinents for storage and saddle racks / hooks. Install a furnace and hotwater tank if you want to really load it up. ( I just installed in ours and it was 650 for both online Ebay dealer) If you put in water add a sink to wash hands with and a cowboy shower in the back.
Carpet living quarters area
Put windows in the sections that are open to give extra light inside of the living quarters
Put a mattress in the nose to sleep or rest on.
Mats in the back
hay mangers for the horses
Do you want it to be a slant or strait load? If slant put up dividers so the horses have something to lean on and stablize themselves during the ride.