I've talked before about what I think makes for effective games that keep players wanting more (namely listening to the players and establishing real stakes as described in this blog post), but I recently encountered a Reddit thread wherein the original poster was asking about advice for trying to limit the 5e version of the simulacrum spell. The rule for 5e simulacrum is that a given caster can only cast one instance of simulacrum at any given time, but those simulacra could use their one cast of simulacrum on the caster, creating an infinite army of wizards given enough time and powdered rubies. The redditor in question was worried about this "exploit," and was asking for advice.
While a lot of advice in the comments suggests, "go ahead and limit it, you're the GM," I don't find that satisfying, because I tend to think the inflection points of a thing are usually the most interesting part. So in keeping with establishing real stakes, here was my response. A specific example that is potentially broadly applicable; when your players can do something wild and creative that seems like it will upend your carefully crafted world, let them do it! But also, brainstorm the ripple effects this action will have. Nothing happens in a vacuum, and the world keeps moving while the players wait.
(Also, part of the joy of being a powerful wizard in any role-playing game system is doing wild, impossible things. Don't take that from your players!)
I replicate my response below:
As the DM, you are perfectly within your rights to limit aspects of the rules or setting. That's technically the end of the discussion, although I might canvass the entire player group and see what they think.
But personally? I wouldn't limit simulacrum. I'm usually in favor of whatever nonsense my players want to do.
However, there is always a cost.
Each casting of simulacrum costs 1,500gp and takes twelve hours, which means you can functionally only cast it once per day despite your spell slots. (If your characters have folded time or don't need to sleep, then maybe you get two castings if you have the spell slots. Wish obviously changes the calculus of these assumptions, but it does that anyway.)
That means your magic-user's clone army takes weeks to amass. If anything else is happening at the moment, it continues to happen while they're doing this. If the other players don't want to wait, then they get to go have adventures while the wizard sits in their laboratory and makes simulacra all day.
And what about ruby dust? Your player might have plenty of rubies (a single "typical" 5,000gp ruby as described in the treasure portion of the Dungeon Master's Guide yields enough powder for 3 1/3 castings of simulacrum), but will probably have to get more to amass an army over the course of months. That means they have to go delving, or mining, or trading. How many rubies are in a typical settlement, anyway? A typical mine? Is the queen willing to pry every ruby out of her crown to power your simulacra?
All of this to say: the spell is not instantaneous, and it's not without cost. Casting simulacrum takes time and resources, the sort of large-scale project that someone notices and tends to engender strong opinions. What do the NPCs do when they realize a wizard is casting simulacrum over and over again? What do they do when they realize a wizard has 50 rubies on hand? (Worth at least 250,000gp; the GDP of whole city-states.) What do the other player characters do when they realize the wizard is going to spend the next two months blasting through their wealth to build clones?
Are things so quiet in the world that the characters can afford to wait while the wizard performs their Great Work in their ivory tower? Is there no political or religious organization in your setting that would be very interested in these activities? Is the villain merely sitting idly while the players gather a magical army vulnerable to antimagic?
I don't know the parameters of your campaign. But I do know that player decisions don't happen in a vacuum, and in high-level games, they have consequences. Big consequences. What are the consequences of this choice?
Play to find out.
