The tiers could be like this:
Grounding I:
A player struck will be unable to fly for 1 second. A flying creature struck is slowed for 1 second.
Grounding II:
A player struck will be unable to fly for 2 seconds. A flying creature struck is slowed for 2 seconds.
Grounding III:
A player struck will be unable to fly for 3 seconds. A flying creature struck is slowed for 3 seconds.
This would help to keep players with elytra from having a fairly simple method of escaping from dangerous pvp situations and slightly reduce the dominence of elytra in pvp combat.
Grounding I:
A player struck will be unable to fly for 1 second. A flying creature struck is slowed for 1 second.
Grounding II:
A player struck will be unable to fly for 2 seconds. A flying creature struck is slowed for 2 seconds.
Grounding III:
A player struck will be unable to fly for 3 seconds. A flying creature struck is slowed for 3 seconds.
This would help to keep players with elytra from having a fairly simple method of escaping from dangerous pvp situations and slightly reduce the dominence of elytra in pvp combat.