I saw this on a stream and it got me wondering. Instead of attacking with the spiritual weapon spell, they were allowed to try a shove using their spellcasting modifier rather than Athletics, contested like any other shove check.
What are the balance implications of such a change?
"Spellcasting Modifier" is an informal term that means either "spellcasting ability modifier" alone, or "your spellcasting ability modifier + your proficiency bonus + any special modifiers". Consult your DM.
Typically casters have a higher Spellcasting Modifier than they have Strength (Athletics) modifier. This means that allowing players to use their Spellcasting Modifier when shoving buffs casters.
What are the exact balance implications?
The usefulness of Shove is quite niche. It is effective at what it does, but how often it can be useful will depend on your campaign. Thus any changes could have no effect whatsoever, or they could be very useful.
From a DM perspective I think it is a reasonable and thematic change for when a caster has a plan in mind.
Please note that even without any changes to Shove, the caster could still ask the DM "hey I know Spiritual Weapon specifies an attack, not an attack action, but could I try to knock them prone/push 5ft as per Shove instead of attacking?" since the rules say:
When you describe an action not detailed elsewhere in the rules, the DM tells you whether that action is possible and what kind of roll you need to make, if any, to determine success or failure.
While most players wouldn't complain, especially those in melee getting advantage on attacks, some might not like it since casters are already pretty powerful.
If you allow this, then you are now giving the caster a ranged shove as a bonus action that doesn't require Strength.
So now the caster doesn't have to worry about getting into melee range, no Opportunity Attacks to worry about, and they still have their action. Compare that to melee classes that have to give up one of their attacks to make the Shove action, or take a feat to be able to do it as a BA, and even then that is only after they have attacked.
A concern in terms of "balance" is that the Cleric now no longer needs to make a choice in terms of boosting strength or not, since they will already tend to boost the spell casting ability. This is an 'inside the party' balance concern in terms of opportunity cost.
"Spellcasting Modifier" is ability modifier + your proficiency bonus + any special modifiers
Allowing players to use their spellcasting modifier when Shoving, as you propose, means they get a boost to what amounts to an Athletics check without having to invest in the Skill, nor having to invest in Strength.
The recent ruling by Crawford (SA Compendium, 2019) that the Shield Master's shove happens after the attack Action by the Fighter/Paladin/Barbarian/Whatever (which reversed the original ruling of "either before or after"(SA Compendium 2016)) was informed by the potential for "Shove, then two/three attacks with advantage" combination that exploits the Prone condition (Appendix A) yielding advantage to melee attacks within 5'. Having played a champion with the Shield Master feat, and three attacks per round, I noticed the difference in my damage output when my DM went with the second ruling. I had to get more clever in setting up my Barbarian ally with proned enemies to attack - he had the Great Weapons Master feat, so we tried to do that where we could.
A Spiritual Weapon bonus action Shove, combined with a first level melee spell attack, can yield a spike in damage potential. The normal roll for Shove takes an action. By using a bonus action to Shove all other Actions are freed to follow the Shove.
Example ~ Inflict Wounds
If cleric can Shove with a bonus action and then attack the prone enemy with the first level spell Inflict Wounds then cleric has advantage on a spell attack that does 3d10 necrotic damage. The choice to use that spell becomes far less risky: as that spell works now, it's an all-or-nothing chance on a single roll. Cleric now gets two rolls. Advantage's nominal benefit yields "+ 4 - + 5" to an attack that powers this up considerably.
Compare the chance to Hit Versus AC 14; Wisdom of 16 at level 3:
I have to roll a 9 to do 3d10 damage regularly, versus I have to roll a 4 or 5 with advantage (based on the chance improved by advantage). Also note that my chance to roll a critical hit (for 6d10) goes from 5% chance to just over 10% chance. (Yes, cleric can still miss).
Example 2 ~ Spirit Guardians(SG)
A Prone creature has to spend half of its speed to get up, and then if it wants to get out of the radius of SG's 15' radius it has to move with its movement already halved - save or not. (Granted: the attempted Shove is not a guarantee).
An affected creature’s speed is halved in the area, and when the creature enters the area for the first time on a turn or starts its turn there, it must make a Wisdom saving throw. On a failed save, the creature takes 3d8 radiant damage (if you are good or neutral) or 3d8 necrotic damage (if you are evil). On a successful save, the creature takes half as much damage. (Basic Rules, p. 105)
Being able to use a bonus action to knock them prone, and then stand/move next to them with Spirit Guardian up (and beyond that to Dodge as an action on successive rounds so that the chances of losing concentration by being hit goes down) will subject all but the fastest enemies to at least two turns in SG - maybe more if the Cleric goes for a "rinse and repeat" Shove to knock prone on subsequent turns. Granted, SG's major benefit is area damage rather than massive damage to a single creature, but this combination increases its lethality for no action economy cost. A cleric could also try on the second turn to Grapple (as an action, Athletics Check) and turn the creature's speed to 0, which makes moot an attempt to get out of Spirit Guardian radius.
Let's see that again: Bonus Action to Prone, Action to Grapple, and the speed is 0. No chance to stand up until breaking the Grapple. The prone enemy is stuck in the SG radius for a bit longer. (Yes, there is no guarantee that the Grapple succeeds)
Example ~ Shove and run away.
By using the bonus action to shove, both action(Dash) and Movement can be applied to a retreat. If the Shove knocked the enemy prone, its Opportunity Attack(OA) is with disadvantage. If the Shove moves the enemy back 10 ', no OA. (I actually like this one, but it's not the only combo that can be applied).
Example ~ Shove prone a flying enemy within range; falling damage.
As the rules on flying creatures stand, a proned flying enemy begins falls.
{SG} Range: 60 feet (Basic Rules, p. 106)
The falling enemy takes 6d6 damage, and is prone, and on the ground until it can try to get up. The Cleric's allies probably make hash out of it, and if it fell within the radius of Spirit Guardians, see above ...
Flying Movement
If a flying creature is knocked prone, has its speed reduced to 0, or is otherwise deprived of the ability to move, the creature falls, unless it has the ability to hover or it is being held aloft by magic, such as by the fly spell. (Basic Rules, p. 74)
Don't do this. The action economy expansions that I mention are hardly the only ones that someone will find. Spiritual Weapon is already a very nice spell since it does not require concentration to keep up, and, you can move it around the battlefield. It's a "must have" for all clerics that I play.
Go for it! Clerics being able to abuse enemies in new and interesting ways, particularly as they go up in level and don't get extra attacks, is fun for the whole table.
Let the Wrath of Umberlee (in the form of a proning Spiritual Weapon) knock them to their knees where they can beg for her mercy!
