First off, I know grenade throwing mechanics are getting changed. This is not about that.
I've noticed in this game, the explosive grenade behaves basically like a concussion grenade.
These are the types of grenades that use a sealed container to produce enough pressure to make a strong shockwave, with enough energy to damage living tissue and organs.
These grenades have a relatively small effective radius in open spaces, and excel when used in contained locations such as bunkers or rooms, where their effective killing range is boosted.
These are also extremely effective underwater (as opposed to supersonic bullets, which shatter just a few inches after they break the surface).
Due to water being pretty much incompressible the shockwave in an underwater detonation gets transfered with much better efficiency into your innards, causing tremendous amounts of internal damage. This makes aconcussion grenade underwater more lethal and far reaching than on land.
On the other hand, fragmentation grenades such as the infamous pineapple (mk II) grenade, work not by creating a very strong pressure wave, but by using a weaker pressure wave to propel shrapnel in a radius around the explosion. This gives a fragmentation grenade a larger range as well as increased effectiveness in open spaces, but reduced efficacy against protective gear which can catch shrapnel (steel plated vests, helmets).
I think that given this game's semi realistic approach, we could have both. Frags would need to emit (relatively) short ranged shrapnel in a radius (essentially, use bullets with the existing ballistics system ingame, tweaked for proper damage and range).
Concussion nades would basically behave, like the grenades we have now, tweaked for increased efficacy in both water and enclosed spaces, and decreased efficacy in open environments.
This would allow for a choice of lethal grenade according to a tactical situation, either clearing a building, or trying to grenade a player in open environment, and could open a nice opportunity to balance grenades for both these situations without making them overpowered in either scenario.
Both of these could be easily grasped by even new players with a simple tooltip ("Emits a shockwave. Best in closed spaces", "Creates shrapnel. Best in open spaces"). It would also add a touch of realism to grenades, and further their tactical potential.