- Be situtational if they are powerful.
- Have a meaningful resource cost.
- Pose a trade off to the player.
(My personal opinion is that some equivalent abilities - e.g. the Order Domain feature - do not abide by these restrictions, but obviously there are contextual differences between subclasses).
I've written a new homebrew replacement for the War Priest feature based on that feedback. It allows a War Cleric to make an attack when they reduce the power of a spell they cast.
When you use your action to cast a spell by expending a spell slot, you may instead have the spell assume the level below the slot's and use your reaction to make one weapon attack against a creature that was a target of the spell.
The assumed level must be a valid level at which the spell may be cast.
It's situational. It requires the cleric to be blasting / debuffing (instead of the more common healing / buffing). This also better matches the aggressive nature of the original feature.
It's limited by the number of spell slots the cleric can use, and costs a reaction. There is a soft precedent for using a reaction on your turn in the UA Blade Mastery feat, and there may be other such features that I'm not aware of.
It trades an extra attack for one level of a spell slot. So the cleric can either throw away a low-level slot to weak blast + attack, or nerf a high-level slot to strong blast + attack.
It stacks with Spiritual Weapon. So in the best case, the cleric can blast + reaction attack + bonus attack with spritual weapon. In contrast, the Order Domain cleric can heal + have an ally reaction attack + bonus attack with spiritual weapon.
Do you think there are mechanical or balance issues to the homebrew feature?