In our last session (with me as the DM), a creature was surrounded by the party and was trying to flee. She cast Levitate on herself to try to float up and get away. As she was going up and thereby out of reach of several party members, they each got an Opportunity Attack against her. Several of them wanted to use this opportunity attack to try to grapple, holding on to her to keep her from getting away, rather than a more traditional melee attack. I wasn't sure whether this was an allowed way to take an Opportunity Attack, but the scene of her floating up with one of the player characters grabbing onto her legs as she went up was pretty neat, and I figured via Rule of Cool that I should just allow it. And it did lead to a really fun encounter, with the character dangling off the legs of this creature in midair trying to slap some manacles on her to prevent further spellcasting.
Now, after the session, I figured I'd look up the actual rules, and sure enough by the book Opportunity Attacks only allow for a melee attack, not a grapple. So I guess my question is, what issues might I encounter if I maintain this precedent of allowing a more generous set of actions as Opportunity Attacks? It sure seems that trying to tackle or trip somebody as they run past you could make for some fun scenes, and it feels more "realistic" in some sense than only allowing for a melee attack.
I'm a relatively novice DM, and am thus hesitant to go very far outside of the standard rules.
If I just allow for this -- substitute in a Grapple for a melee attack during an Opportunity Attack -- will this create balance problems?