Tony Stark, Nathan Stark & Xander are all at least half brothers, with Nathan and Tony having known each other previously.

Xander runs away from home as a teenager{14-17(due to abuse, backstabbing, injury...all of the above...)} and runs into Tony, ends up saving his life, which eventually leads to them finding out the family connection.

Tony and Nathan join together to help and get to know Xander, ensuring he gets extensive, effective & excellent Healing, Therapy, Tutoring and Self Defence Training{in case he gets kidnapped}

Slowly they each form a strong and solid accepting/protective/loyal/resiliant/ perceptive/ healing/in-depth relationship with each other - in the case of Tony and Nathan it also means healing at least some of the personal hurts/issues with themselves and each other 

No one outside of the brothers and JARVIS know of the family relationship or who Xander was previously, and they try and keep it that way even after the death of Stane.

All three are skilled BAMFs, Engineers, Business Men, Humanitarians, Façade Wearers, Secret Finders & Keepers{includes the Supernatural world}, Fighters, Linguists, People Readers... 

Bonus points if all three are natural shifters, hunters, sentinals/guides, immortals or psychics...

Double bonus points if all three are either gay or bisexual {possible lovers Jack Carter, Phil Coulson, Clint Barton, Bruce Banner, Jack Harkness....}

Triple bonus points if sometime between sunnydale and meeting tony Xander somehow looses his memories

Quadruple bonus points if none of the Starks are in awe of Steve Rogers and in fact have issues with him

Just imagine Fury reaction to learning there are three Starks running around :D

Xander as the brother of Nick Burkhardt that was given up by his aunt Marie after the death of his parents and then fostered by the Harris's for an extra paycheck.

Must Have

= Xander having a close long term protective friendship/mentorship with a good Wesen couple who ensure he turns out to be a good, open minded, down to earth, self confidant man despite the daily negative influences of the Harris's, his classmates, teachers and the Scooby's.

= Xander having won several competitions in Archery, Swimming, Running, Art... thanks to his positive role models and to the displeasure of Willow.

= Xander having his own detailed, disguised, encoded, modern and frank Grimoire {think Charmed Book of Shadows in size/look with a lock} giving the full lowdown/profiles of everything & everyone supernatural {not just the Wesen} without Bias. Includes Homemade Remedies to various conditions, injuries, ailments, poisons & magics.

= Competant, Observant & Artistic Xander with necessary life skills thanks to his mentors > Self Defence & Daily Fitness, Profiling, Languages, Problem Solving, Camp/Hunt/Track/Fish, Handyman, Mechanic, First Aid, Homemaking{Cooking/Housekeeping/Maintenance/Management}, Research...

Xander finds his brother whilst in his teens and they form a close unbreakable bond, particularly when Nick finds out how his brother has been treated and takes him in, moving him to Portland, further ensuring that Xander grows up a good man. 

Xander helps provide Nick with a Solid/Thorough education in the Wesen/Supernatural world pre Grimm so he's not as lost/freaked out when he does become a Grimm  

Xander as the elder half brother of Harry Potter, who has had a comprehensive magical and mundane education, unbeknownst to anyone in Sunnydale barring Jesse McNally who was a First Generation Wizard.

Special Magical Talents = Shamanism, Runes, Warding, Combat Magic, Magical Craftsmanship & Enchantments, Wolf Animagus, Mind Shield, Rituals

Also has a large trust fund & a fully secure Manor house in Colorado Or Wyoming that no one has access to outside of himself.

At age 17 he graduates both from his Magical & Mundane schooling & is accepted as an Auror

When the Dursley family is killed, he gains full custody of his half brother.

Helps Harry, Remus & Sirius get a Full = Examination, Cleansing, Physical & Psychological Healing, Private Mundane & Magical Education Tutoring, Restraining Orders, Inventory & Investigation Of All Properties & Assets By Blood-Magic-Law(e.g. Lineage, Bequests, Gifts...etc),

Wizarding Britain ends up getting a comprehensive, detailed & systematic investigation in their government, education, economy, medical...etc

And when the Scooby Gang find out and try and interfere, they end up being fully investigated, cleansed, physically & psychological healed, educated on reality & trained/backed up properly in combat if they are not arrested themselves

Extras

Bill & Charlie Weasley transfers even further away from England,

Percy Weasley decides to become a ICW Lawyer instead of becoming British Ministry toady,

The Twins become R&D Scientists with a private Canadian company with their dream Joke shop as a side business,

Ron Weasley enventually becomes a Oceania Auror after his attitude is worked out of him,

> all of them have minimal contact with their overbearing banshee mother or doormat father & continually refuse to go back to England

Hermione is continuasly shown the error of her ways such as in daily-chan fanfiction,

None of the Hogwarts staff are allowed to get away scott free as they are not innocent bystanders

No Tonks/Remus relationship as she's too immature/naive for him

Both Xander & Harry get adopted into Black Family