The best replacement for slacks is trousers.

The best alternatives to slacks are rated as following:
★★★★★  trousers
★★★  jeans
★★★  pants
★★★  khakis
★★  dress pants
★★  chinos
★  suit pants
★  flat front
  khaki
  cargo pants

Your alternative for slacks is not listed? Suggest!
Speaking about slacks, one probably shouldn't forget about polo shirt.

Other search terms related to slacks and their ratings are:
★★★★★ polo shirt
★★★★★ brown shoes
★★★★★ untucked shirt
★★★★ cowboy boots
★★★★ polo
You have in mind a companion for slacks and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of slacks in all years. Click on graph for more.
slacks
trousers
jeans