As discussed somewhat in Is knocking someone prone worth it? the advantages of knocking prone are rather situational, and in particular, depend a lot on where you and the enemy you want to knock prone fall relative to one another in the initiative order.
If I knock someone prone whose turn is right after mine in the initiative order, the only impact that has is using up half of their movement. Considering they're right next to me, they probably didn't want to move anyhow. They're going to get right back up and hit me, with really no ill effects.
On the other hand, if I knock someone prone whose turn is right before mine in the initiative order, they're down for almost the entire round, and my nearby companions may have the opportunity to take melee attacks against them with advantage.
I would like to somehow add a house rule that reduces the initiative-order dependence of the benefit of knocking prone, but I'm not sure what would work well.
I considered making getting up from prone provoke attacks of opportunity, as it did in 3.5e (and maybe other editions) but that's probably too much of a penalty in the context of 5e combat.
Does anyone have a playtested house rule that makes knocking someone prone more consistently worthwhile (at least if you have nearby allies)?