“Any system can do anything you want it to do.” — from TTRPG Twitter.
I’m leaving the name off of this because I’m not trying to cause problems in the community. This person is technically correct. But in the interest of discussion, I will say there is a larger continuum to consider here.
From a tactical or strategic wargame perspective, no. Absolutely one system doesn’t work for everything. Typically, many RPGs don’t translate well to wargames/miniatures warfare. Ironically D&D came from the miniatures game Chainmail, which was a wargame. But if one tries to run epic naval battles with D&D 5E, for instance, it’s going to come up short on a mechanical level. One could just as easily create an entirely new game in the amount of time it would take.
Mashing the medium round peg into the medium square hole.

When it comes to RPGs, yes one can make any system work for just about any game. Yes, you can play virtually anything from stone age fantasy all the way through supers in space with D&D 5E. It’s possible because roleplaying doesn’t require some of the crunchier nuances that wargaming requires.
The whole thing comes down to how much time one wants to spend converting the game to work for one genre to another. How many hours does it take to rework D&D 5E into Call of Cthulhu in the 1920’s? Would it be easier just to buy another game? Would it be easier just to grab a set of more generic, universal core rules to do the same thing?
Yeah, it’ll fit with enough force.
Some game systems hold up to being manipulated better than others. D20 is the most common and debatably popular system as a core on and off for the last 20+ years. But it’s not always the fastest or most efficient when it comes to converting it into specific niches. For example Mutants & Masterminds looks almost nothing like D20 Modern, even though they’re both based on the same SRD/OGL.
I fall back on FATE and FUDGE for a lot of the quirky one shots or mini campaigns I come up with for certain niches because the conversion is relatively idiot proof. Their dice mechanics are simple and flexible for everything, especially combat. Character creation is pretty much the same from one genre to the next with a few minor adjustments. (FATE Horror and any game with supers takes a bit of tweaking.)
I will say that DriveThruRPG and similar websites offer a ton of options when it comes to generic systems. I’ve found a lot of gems such as Fantaji and GMD Core on there. Savage Worlds, the system that Deadlands RPG runs on, is also available. It is a good, crunchy generic system that has been adapted to fit several campaigns in multiple genres. That’s also where I discovered ICRPG which is exceptionally adaptable.
Time to get out the left handed monkey wrench.

So, it’s either spend potentially hours or days converting a d100 or d20 RPG into whatever genre or game you want. Depending on the complexity of the game one desires, the amount of crunch the players are going to want, and the specific mechanics for some settings (horror, for example.) OR one can simply grab a generic core system and have the whole thing knocked out in an hour or two with some minor adjustments on the fly. Some games are intended to scale into one size fits all.
At then end of the day, it’s a matter of how much time you as a GM and your players want to spend haggling over character traits, historical data, combat mechanics, scale, and dozens of other factors. Personally, I like to get the right tool for the right job. If a preexisting game covers the bases, I’ll grab it and use it. Your mileage may vary.
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