The best replacement for fatboy is road king.

The best alternatives to fatboy are rated as following:
★★★★★  road king
★★★★  softail
★★★  fatboy lo
★★★  heritage
★★  dyna
★★  fat bob
★★  breakout
★★  deluxe
★  softail deluxe
★  street glide
★  wide glide
★  heritage softail
★  sportster
★  lovesac
★  farley
★  sumo
★  moonlander
★  v rod
★  jumbo versipack
★  roadstar
★  street bob
  roadking
  fatbob
  lowrider
  fatboy jr
  heritage classic
  bazaar
  night train
  victory kingpin
  slim adjustable
  switchback
  deuce
  xforce
  kingpin
  superglide
  bazaar bag
  yamaha roadstar
  fatboy low

Your alternative for fatboy is not listed? Suggest!
Speaking about fatboy, one probably shouldn't forget about hard saddlebags.

Other search terms related to fatboy and their ratings are:
★★★★★ hard saddlebags
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Google Trends

Scale is based on the average worldwide traffic of fatboy in all years. Click on graph for more.
fatboy
road king
softail