Blizzard has stated that players will gain about 40% more experience while leveling in Diablo 4's upcoming Season 2!
Diablo 4 Season 2 Liveblog
As Joe Shely has explained in the Diablo 4 Season 2 Developer Update, all experience changes coming with Season 2 — most importantly, World Tier experience bonus becoming multiplicative — will, cumulatively, increase player experience gained by up to 40%.
Shely has also commented that this change is meant to speed up the general leveling process to make reaching Level 100 an «achievable goal» as part of Season 2's Season Journey and so that players will be able to experience Season 2's new content such as new pinnacle boss fights.