What balance problems arise by allowing grappling as an option for an Opportunity Attack?

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?

How does Freedom of Movement interact with the “Move” action while grappling?

The question requires a two-part answer:

  1. Does a character affected by Freedom of Movement need to make a successful grapple check to use the Move action, or would success be automatic?
  2. If successfully using the Move action, is a character affected by freedom of movement able to move the grapple by its full movement speed or only half?

Move
You can move half your speed (bringing all others engaged in the grapple with you) by winning an opposed grapple check. This requires a standard action, and you must beat all the other individual check results to move the grapple.

Note: You get a +4 bonus on your grapple check to move a pinned opponent, but only if no one else is involved in the grapple.

The main issue to consider is how the Freedom of Movement effect works on the other characters in the grapple. Is the only bonus granted in a grapple the ability to simply leave the grapple whenever desired, or does it work more like encumbrance reduction and allow the character to move at full speed (dragging others along) when it would otherwise be unable to do so?

Using a “grappling hook” in combat

So my character has the ability to use the Levitate spell, and while useful, it does require me to push or pull of an object or surface. To make this easier I thought about using a “grappling hook”. Something simple, like a 20ft length of rope, with a hook on the end. This could make moving around much easier.

However, I was wondering more specifically about it’s use in combat. The tricky part is that my character is dex based, not str. 14 dex and 10 str. So it’s use would be to simply attempt to throw an enemy off balance, or impose disadvantage on their next roll, for example.

The use in this situation would have to be finesse based, potentially, or the goal would be an opposed check vs the targets dex (and my dex, hopefully).

Is there anything in the rules about something like this? Or is it entirely Homebrew?

What are the grappling rules for the caster of the spell Entangling Staff when it successfully grapples a creature?

The spell Entangling Staff (Spell Compendium p.83) states that once you are grappling from the effect of the spell:

If your grapple check succeeds, your quarterstaff’s vines constrict your foe, dealing 2d6 points of damage (you can choose to deal nonlethal damage instead of normal damage if you wish). You then have two choices:

Release: You release your opponent from the grapple. Some vines remain clinging to your foe, leaving it entangled for the duration of the spell. You can attack different enemies in later rounds with the staff, potentially grappling and constricting or entangling them.

Maintain: You maintain your hold. In subsequent rounds, you deal constriction damage with a successful grapple check. You can then choose to release or maintain the hold again.

Does this spell limit the options available to the caster while grappling, or does it add options?

For example, if a hill giant druid uses this against a party and establishes a grapple, does that hill giant have to specifically roll a grapple check to do damage with the staff, or does the staff simply add constriction damage to regular grapple checks effectively giving the creature a lesser form of constrict?

If the giant must roll specifically for the staff to constrict, are multiple rolls allowed based on the creature’s remaining iterative attacks, or can it only make a single check per-round due to the wording of the spell?

Ruling for Grappling a Creature Underwater While you are on Land?

If a character is on the bank of a river and they want to reach into the water and grapple a creature in the water (assuming that they are within the size limit to be grappled) are there any special rulings for grappling or is it the standard grapple check?

According to the Grappling Rule as listed on Roll20

Grappling

When you want to grab a creature or wrestle with it, you can use the Attack action to make a Special melee Attack, a grapple. If you’re able to make multiple attacks with the Attack action, this Attack replaces one of them. The target of your grapple must be no more than one size larger than you and must be within your reach. Using at least one free hand, you try to seize the target by making a grapple check instead of an Attack roll: a Strength (Athletics) check contested by the target’s Strength (Athletics) or Dexterity (Acrobatics) check (the target chooses the ability to use). If you succeed, you subject the target to the Grappled condition (see Conditions ). The condition specifies the things that end it, and you can release the target whenever you like (no action required).

But when you look at Under Water Combat on Roll20

Underwater Combat

When making a melee weapon Attack, a creature that doesn’t have a swimming speed (either natural or granted by magic) has disadvantage on the Attack roll unless the weapon is a Dagger, Javelin, Shortsword, spear, or Trident. A ranged weapon Attack automatically misses a target beyond the weapon’s normal range. Even against a target within normal range, the Attack roll has disadvantage unless the weapon is a crossbow, a net, or a weapon that is Thrown like a Javelin (including a spear, Trident, or dart).

It appears there is no clear ruling and that the standard grapple check applies. Is that correct?

Does grappling an Animated Object let you cast Shillelagh on it?

I was looking at shillelagh whose description states (emphasis mine):

The wood of a club or quarterstaff you are holding is imbued with nature’s power… The spell ends if you cast it again or if you let go of the weapon.

And then looking at animate objects whose description states:

Each target animates and becomes a creature…

Shillelagh ends when you let go of the weapon (which is now a creature) and the rule on “Moving Around Other Creatures” states:

Whether a creature is a friend or an enemy, you can’t willingly end your move in its space.

In order to cast shillelagh then, you would need both of the following to be true:
You are holding the animated object.
You are not in the same space as the animated object.

One way I thought this might be possible is if you grappled the animated object, so that you are not in its space but arguably holding (not wielding) it.

Does grappling an animated object let you cast shillelagh on it?

Can an ankheg make an opportunity attack while grappling?

The Ankheg has one single attack action (aside from its acid spray):

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 3) slashing damage plus 3 (1d6) acid damage. If the target is a Large or smaller creature, it is grappled (escape DC 13). Until this grapple ends, the ankheg can bite only the grappled creature and has advantage on attack rolls to do so.

So if it attacks someone with a bite and hits, it grapples them and can’t bite anyone else.

What happens if someone else moves out of its attack range? Can it not make an opportunity attack because it can’t use its Bite ability? Or does it make an “unarmed” attack?