The best replacement for 802.11ac is 802.11ad.

The best alternatives to 802.11ac are rated as following:
★★★★★  802.11ad
★★★★★  802.11g
★★★★  powerline
★★★★  802.11a/b/g/n
★★★  802.11bgn
★★★  wigig
★★  moca
★★  802.11b
★  802.11n
★  802.11an
  802.11agn

Your alternative for 802.11ac is not listed? Suggest!
Speaking about 802.11ac, one probably shouldn't forget about 802.11n.

Other search terms related to 802.11ac and their ratings are:
★★★★★ 802.11n
You have in mind a companion for 802.11ac and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of 802.11ac in all years. Click on graph for more.
802.11ac
802.11ad
802.11g