Visionaries are odd people. Plagued by ill health and madness from birth, each is a frail, frightened, wide-eyed thing, overwhelmed by the signs and portents they see at every turn. Something in their mind processes the world differently to other people. They see things others don't, understand things in ways other's can't, draw bizarre connections between things. By all accounts, they're totally insane, tormented by hallucinations, delusions and paranoia.
Except, of course, that they're frequently entirely correct. Their insights have a habit of being correct, and their madness has an underlying method to it that might be obscured even from the visionary.
Many such individuals find themselves locked away in sanatoriums and asylums, or kept behind closed doors by families ashamed of the lunatic in their midst. Others go out into the world, becoming vagrants, hermits, or social dropouts. A few manage to find themselves recognised as oracles, kept somewhere safe and consulted for prophecy.
A significant number, however, are recruited as adventurers. Neither front-line combatants, nor skilled problem solvers, a Visionary contributes little directly to their party's success. Any party that can recruit one, however, knows the value of the wild-eyed, babbling prophet accompanying them. When things are about to turn nasty, the Visionary becomes the metaphorical canary in the coal mine, and when the party's course is in doubt, the Visionary's insight becomes invaluable.
Saves: As a Magic User
Attack: As a Magic User
Experience Costs: As a Magic User
Weapon & Armour Restrictions: If your system restricts gear by class, no armour or shields. Weapons limited to one handed weapons, spears, thrown weapons, and slings.
Madness: The Visionary is not even slightly sane, and is not very good at hiding it. The precise details are up to you to decide; they might hallucinate wildly, have weird obsessions, find themselves driven to constantly complete arbitrary rituals, talk to themselves (or to things nobody else perceives), believe things that seem nonsensical or baseless to others, or something else. The exact details will vary wildly from Visionary to Visionary, but all exist as a coping mechanism for the visions they are constantly beset with. While there are no hard-and-fast requirements for their actions, it will always be clear to anybody observing them that they're mad. They can suppress this for a time if they wish, doing their best to put on a facade of sanity, but while doing so all their other Visionary abilities cease functioning, and they cannot gain experience for anything they do or achieve while pretending not to be mad, even if they were only feigning sanity for a small portion of the venture.
Uncanny Insight: A Visionary can, if they concentrate, find themselves knowing all sorts of things that they shouldn't have any way to find out. The information comes to them filtered through weird symbolism and hallucinations, but they can still glean a certain amount of truth from it. To use this ability, the Visionary must be able to concentrate for a few seconds (a full round in combat) to put all the signs together. Roll a d10. On a 1-9, the Visionary gets to ask the GM a single yes-or-no question on any topic, and get a truthful answer. On a 10, the Visionary's insight has run out; they cannot ask about that general topic again until they gain a level.
Foreboding: Visionaries also tend to have a sixth sense for danger, even if they're often unable to articulate exactly what the nature of the threat might be. Whenever the Visionary or their companions are about to do something truly foolish or dangerous, or whenever they face an oncoming threat that they have no knowledge of, the GM should give the visionary a warning that they're in danger (if they want to know what from, they can use Uncanny Insight to find out more). After receiving this foreboding. the Visionary has two options available to them. They can either tune out their danger-sense, losing access to Foreboding for the rest of the day, or concentrate on it, allowing them to benefit again the next time danger looms. Concentrating on their danger-sense is mentally taxing; each time they they do, the chance that their Uncanny Insight fails entirely increases by 1 for the rest of the day. (IE: if they want to keep their Foreboding active after the first time it goes off, they fail Uncanny Insight on 9-10. After a second warning, Uncanny Insight fails on 8-10, and so on).
Second Sight: Visionaries literally see things other people don't, and sometimes those things are really there. They permanently get the benefit of See Invisibility. However, anything they can see in this way is hard to distinguish from mere hallucinations and false-positives; the invisible things get the benefit of the spell Mirror Image, with 4 imaginary duplicates of themselves; when the Visionary targets an invisible thing they can see in this way, randomise whether they hit the real target, or just prove that one of it's imaginary duplicates isn't real (eliminating it), until either the real one is revealed, or else all the duplicates are proved to be imaginary.
Uncanny Reflexes: A visionary is often aware of threats a split second before they begin, and if they concentrate on this, they can be surprisingly hard to pin down in a fight. If they spend their entire action doing nothing but trying to avoid harm, they get +4 AC and +4 to their saving throws against any attacks directed against them; doing this doesn't prevent them from moving.
(It should go without saying that this class models somebody driven mad by supernatural prophecies and visions, and in the real world this is not at all how mental illness works. Keep taking your meds.)