I think just stat them as a normal CK2 character, but give them access to exclusive lifestyle perks - e.g. a present day doctor would be an OP CK2 doctor, a present day engineer would be an OP architect/organiser/siege engineer in CK2, a military historian will get OP as fuck bonuses fighting with/against armies they studied (to simulate knowing in advance what battles happened and how). If we're going Connecticut in King Arthur's court the man basically became a next level wizard so a +2 or even +5 around the board wouldn't be out of place
Also you could go the route of giving them artifacts like the ones you find in the After the End mod. So regular dude goes back in time, but comes back with a handgun. Maybe even give them special culture / religion / retinues to simulate the isekai'd dude being able to make pike & shot bois in an era where gunpowder is just starting to be a thing. You could also greatly expand the tech tree and give an event which gives bonus techpoints to an isekai flagged/traited character, to simulate the character disrupting the world (and most importantly, the world actually changing, as normal characters will eventually catch up to whatever the isekai character is doing in time)
*EDIT
You know, I think the best way to handle the traits would be to give them a general Isekai bonus of +3 around the dot. This puts them above a quick CK2 character but not on a par with a genius CK2 character, which seems about right for someone who has a vague understanding ahead of time of what's occurring but is otherwise fairly average.
Then you can start adding more traits, detailing what kind of lifestyle the person had before they arrived in CK2 land, to make some of their disciplines average with CK2 characters, and others superior even to genius CK2 characters.
- Narnia Syndrome +3 stewardship +3 diplomacy +3 martial +3 intrigue +3 learning +50 same trait opinion
This character claims to hail from another time and place, where metal birds soar in heavens pierced by man-made towers of glass, and mankind speaks to one another with messages carried through the air. Despite their imaginative ramblings, they do appear to know a great deal of things long before they occur, and so their idiosyncrasies are tolerated by their Lords and Subjects.
- Police Officer -3 stewardship -2 learning +1 diplomacy +1 martial +3 intrigue +1.0 health +20% chance to arrest -5% revolt chance +3 personal combat skill
Bobby James had been a uniformed civilian for most of his adult life, keeping the peace and upholding the law as a stellar, if unaccomplished member of the boys in blue. Whilst intervening in an altercation on fleet street, a thug struck him once with a hammer on the head, knocking him out cold. When Bobby James awoke he was looking at a burly Norseman of the Varangian Guard, being dragged before the Baseilios of the Roman Empire, demanding he explain how he infiltrated the palace at Constantinople.
Police officer makes for a pretty weird enforcer of law playstyle, so they get bonuses to keeping the peace with the peasantry, spotting plots & arresting plotters. They get modest bonuses to diplomacy and martial, but have no experience in the administration of fiefdoms. Also a modest bonus to personal combat, to symbolise the police being above-average at CQC but nowhere near as practiced as a viking, crusader, mujahid or adventurer at medieval CQC.
- Medical Technician -3 diplomacy -3 martial -2 intrigue +1 stewardship +5 learning +3.0 health (could also give them renowned physician, or mod disease events to give medical technician a similar or superior buff to saving patients' lives).
Aneesh Gupta had been a medical technician since graduating 8 years ago. She hated interacting with patients but still wanted to put her brilliant intellect & prodigious medical education towards the cause of helping others. Running her laboratory tests to diagnose illnesses allowed her to do just that, all without ever having to be sneezed on by a single patient. This Thursday however was proving to be the most painful day in her life. A crazy man in blue robes had paid her Principal Investigator (in gold coins!) to investigate some fluorescent paint, a total waste of time. After checking to see if this thing was radioactive, Aneesh was surprised to see it glow brighter, almost reaching a painfully searing iridescence... When the blue spots cleared from her vision, she looked up to see a very shocked family in ridiculous garb, all crouched around a very sick looking child.
Wait, did that child have the plague?
Insane health bonuses & learning for the accomplished and academic medical technician, balanced out by average scores everywhere else caused by a lifetime spent in antisocial study. Heavy learning/health/prosperity based playstyle, probably really fun to have on hand during THE plague. It'd be pretty funny to have an event where the isekaid technician actually saves the world from the plague.
- Corporate Climber -4 martial -4 learning +1 diplomacy +3 stewardship +4 intrigue -100 Ambition Opinion +30% national tax +25% fertility +10 attraction +0.5 monthly prestige +2% revolt chance
Doge Alessandro Contarini threw his goblet out the closed window in rage, the fine venetian glass splintering and shattering to the ground in so many shards of ruby red and emerald green. The Knights of the Corporate Charter had already breached the outer gates and would break into his personal mansion any moment now, seizing him and what remnants of his family were yet to flee. He considered taking his own life then and there, but curiosity gripped him to stay. What demon could destroy Amalfi, Genoa and even Venice so quickly?
The Knights of the Corporate Charter dragged him before their Chief. It smiled a perfect, ivory smile.
'Good afternoon Doge, we are both here to discuss the terms of our merger and hostile acquisition. It is very simple - you will keep administration of Venice as a new branch of our ever-growing family of franchised brands, and in return you will give us a lump-sum of 1,903 ducats, in addition to 30% of all taxable income generated from your branch's revenues. Your interests will be protected as you gain the opportunity to join the world's fastest growing enterprise of welcoming individuals! What do you say, sport?
The doge sighed in defeat, and signed the papers. 'You win. But why 1,900 and 3 ducats?'
Its smile hurt Doge Alessandro to look at. 'Because you have 1,903 ducats.'
High stewardship & intrigue, but an incredibly poor choice to make the spymaster or steward because of their massive penalty towards anyone who owns anything they want. An ideal candidate for seizing control of the silk road and all the merchant republic trades, but not terribly interested in fighting or learning, both of which interrupt time spent on seizing inglorious wealth. High functioning sociopath with a great deal of attention to self-appearance, confidence and squeezing money from all! Not popular with the peasantry.