I'm working on a small system to use for a Halloween supernatural thriller campaign. I wanted to add character options for some players to have supernatural abilities. My worry is no one will want to NOT have supernatural powers if they can just CHOOSE to have them instead. There's is almost no magic outside of supernatural abilities. Gadgets exist and things like Holy Water, but they're very limited in usage.
What can I do to motivate players to pick non-supernatural origins?
Consider including some sort of drawback to whatever supernatural options you're offering your players. Make it a high risk, high reward choice and some players will probably choose the safer, non-supernatural option. The cost should depend on how powerful the abilities are, but here are some options:
The character takes damage each time they use their ability.
A straightforward option. Perhaps each time the ability is used, the
character takes 1d12 necrotic damage. It could be more or less, depending
on how powerful the ability is.
The character takes a level of exhaustion each time they use their
ability.
I wouldn't recommend this unless the abilities are very powerful, as
exhaustion levels are no joke. Still, it will certainly make them think
twice before choosing a character with supernatural abilities.
Using their abilities attracts monsters.
Perhaps extra-planar monsters are attracted to supernatural abilities. Let
the players know this, and each time they use a supernatural ability, roll
a die behind the scenes to see if they've attracted something dangerous. A
roll of a 1 on a d20 might be appropriate for sparking an encounter, but
you could increase or decrease the odds to whatever you think works.
There is a social stigma against those with supernatural abilities.
If you prefer a role-playing penalty instead of mechanical, this might be a
good option. Let your players know that people are suspicious of or even
openly hostile toward those with abilities. Have NPCs treat those with
supernatural abilities coldly, refusing to cooperate with them. This will
encourage the party to have at least one non-supernatural spokesman.
The abilities come with weird or random side effects.
Maybe these supernatural abilities are beyond the user's control. Have
them roll on the Wild Magic Surge table from the Sorcerer class (PHB 104)
for a random side effect each time they use their ability.
These are a few possibilities, but any drawback could work as long as the price the player has to pay matches the power level of the ability they're gaining.
Some players are inclined to be ornery, or like "hardcore" play, or just like playing "normal people" and will choose to play non-magical characters. You'll have to decide if any of your players are these kind.
Other players like to keep their characters simple to play and choose to avoid abilities like spell casting.
The usual game design approach is to have an option during character creation that provides multiple choices. Some of those choices provide supernatural abilities and others provide different abilities that are roughly as useful. Kind of like the mechanics for the various Backgrounds.
Apply that same idea to your option. I'm going to call this "Destiny". Each Destiny has a benefit - many of those benefits are magical abilities, limited spell casting, and similar, but there are a couple of seemingly mundane abilities for a non-supernatural Destiny - and those benefits are all roughly equal in power. Players choose a Destiny, and get to choose if the character has magical abilities.
The specific abilities, and how they balance, is beyond the scope of this question. Good luck!
I don't know your reasoning for wanting them to chose to not have supernatural abilities. When playing an RPG, players almost always want to have cool powers. Otherwise, they will just be mundane while those around them do cool stuff. It sounds like you have something specific in mind for why you would want non-supernatural characters though.
One way to ensure that some players have supernatural abilities and some don't is to limit their number of choices for class/abilities. If you have 6 players and only 3 classes to choose from, chances are more likely that each class will be used than if you provided 5 or 6 class options, even if 2 of the 3 have supernatural abilities. Along the same vein as "guiding the story" rather than "railroading", this still allows the players to make choices for themselves. Give them options, but limited options makes it clear the direction you would like the story to go without being too overbearing.
This method is used by the RPG system "Dungeon Squad". They give you only 2 class options, Hero or Rascal. It works quite nicely for one-shots.
Want to ensure 4 people will have supernatural abilities and 2 won't? Create 6 character options, let them choose who to play, but only allow each class to be chosen by 1 person. This will prevent everyone from choosing the same abilities and will ensure you have the party composition you wanted. I have used this method in the past with success. Nobody complained about not having more customization options because it was a one-shot.
In my experience with themed one-shots, players don't care too much about having tons of options and don't mind being railroaded a little bit to move the story along. It is more the fun of the storytelling that makes it a fun session, rather than the min-maxing options.
In brief, non-supernatural characters are still good to play, provided that they have their own advantages and extraordinary abilities.
In regular D&D 5e (as your question is tagged), players can choose to be wizards or clerics or warlocks, and bend reality to their will using magic. Or they can choose to be fighters or rogues or barbarians to exercise martial prowess or skill-based expertise. While the spellcasting classes are powerful, the non-spellcasting classes are still appealing to many players, because they present their own strengths.
To look at a few non-magical extraordinary abilities which compete with the supernatural abilities: many martial classes get Extra Attack. Rogues get Sneak Attack. Fighters get Action Surge and Second Wind. Battle Master fighters get Combat Manoeuvres. Barbarians get Rage. Monks get Flurry of Blows. The list goes on. Non-magic users still get lots of cool features, which at least partially balances the lack of spell-casting power.
I had a similar scenario in another campaign I ran, although it was World of Darkness with a homebrew magic system rather than D&D. In World of Darkness, the means to gain abilities (including at character creation) is to spend XP on getting Traits for your character. I had made my magic system part of this Traits system, such that a player needed to buy the Trait which let him be a spellcaster at character creation, then spend more XP on learning schools of magic and spells. However, they need to choose between spending XP on magic Traits, or on other Traits, liking having money, or having ranks in an organisation, or kung-fu skills, or being more hardy, or being a cool gunslinger. In essence, choosing to be a spellcaster represented an opportunity cost, sacrificing other advantages in exchange for having magic powers. Of the three players in that campaign, only one was a spellcaster. Another was a ninja, who had anti-magic Traits and kung-fu Traits. The third player specifically chose to play an ordinary person, and picked Traits which gave them money, status and contacts (social interactions being weighted more heavily in this system than D&D).
In your modern setting, you probably won't be using the base classes as-is, but you should draw inspiration from them. For each supernatural ability you grant a supernatural class or race, give a similarly cool extraordinary non-supernatural ability to the non-supernatural classes and races. You might have gunslingers who can pepper their enemies with bullets. You might have scientists who can pull knowledge from what seems to be thin air. You might have aristocrats who possess great wealth and powerful allies. You might have investigators who can shake off fear and spot even the faintest clues. You might have diplomats who can fast-talk their way past enemies and bolster allies with their words. You could give the non-supernatural races an additional Feat (as the Variant Human in 5e), or maybe some extra skill proficiencies, or perhaps some other benefit which makes sense in your setting.
Your player characters can still be awesome without magic powers, you just have to ensure that the non-magical player character options still present cool abilities.
I would take a lesson from games like Metamorphosis Alpha and GammaWorld (and OSR clones and other riffs) in which the 'pure strain human' has some significant advantage.
For example, a strong dis-belief in the supernatural can ward them against supernatural things (maybe they don't see what everyone else sees, but see a non supernatural equivalent).
Another possibility is the power of innocence and purity. A character could be such an extreme goody two-shoes that the supernatural cannot directly harm them.
