Beginning with the most obligatory one:
*The head will catch fire after taking damage or severing, and extinguish on full repair.
*The head will deal weapon damage to opponents. Try repairing after it falls on or over them!
*The head will bob about on that little spring based on movement.
*When fully repaired, Sweet Tooth will have persistent Suicidal Peds. Someone on this forum had this idea for an ice cream truck.
*There will actually be signs covering the miniguns that will easily break away. It makes more sense for the persistent PUp.
*Textures will be rusty and faded on the outside and blood smeared on the inside.
*The miniguns will spin based on how fast the vehicle is moving, as suggested by BloodstAlexTSK
*Will have sliding doors on the side.
*Rolling door will come loose and lose individual segments with damage, and once loose will roll up and down based on movement
*It will NOT transform into a giant robot.
I'm probably assuming a lot about what's possible in this game. I'll substitute and compromise where needed.
*add windshield wipers
*add dead body and ice cream tubs to glass top freezer
*add magnetic knife rack with knives (and one scoop) over sink (not pictured)
*add Napalm Cone stockpile somewhere in the back
*lots of beveling
*make better wheels
*make undercarriage (suspension, drive train, hubs)
*change ice cream ornaments to soft serve type
*add more to the chair
*misc details if the triangle count is arbitrarily low enough
*set a cash register in the torture chair.