A modder already has smites working per 5e.
On a hit that isnt a crit (with spell slots available) you get a temporary icon to use to smite.
On a hit that is a crit (with spell slots available) you get a temporary icon to use to crit smite.
So i'll be curious to see how larian implements it.
Also the homebrew magic items for pally will be interesting.. maybe temp hp to allies within 10 feet after a smite, and better heavy armor.
Seems pretty easy to smite pop ups on and off as a toggle. Nobody wants to use Smite on a generic goblin -- so it's good to be able to turn that off and not get pop ups if you're just clearing the outside camp. A fight that you may want to toggle the popups on is the warp spiders. Toggle on pop ups incase you're attacking the mother spider, or the spiderlings. The timer on the popup, perhaps 3 seconds long, is a good way to keep things moving.