An AH I have wanted to explore has Harry growing up with Remus and Sirius 'up north' somewhere (Yorkshire?) and accidently have their property back onto the Goyle's separated by a few fields and a stream. Harry as a kid meets Goyle there and they form a friendship, like kids do. Goyle's Death Eater father is up for revenge on Harry, but his mother points out that Harry was but a baby and so had nothing to do with Voldermorts death, plus wouldn't it be an advantage if their son befriended the 'boy who lived?' Goggle snr agrees. Remus and Sirius are horrified at who their neighbours are, but have sunk money/roots/security runes in locally and can't move easily- plus Harry likes Goyle.
Fast forward a few years and Goyle and Harry are best friends, and Goyle has introduced Harry to the family business of training Security Trolls, which Harry quite likes- living with a Werewolf he understands the need for security. The Trolls seem to like Harry in turn, esp. Harry's rather accented attempts at Trollish.
Via the Goyle's Harry is also introduced to 'high society' via parties at the Goyle's manor- inculding meeting the Malfoy's, Crabbe's etc. Sirius and Remus attend to mutual hostility- which is kept to verbal taunting for the 'sake of the children' - though both sides are able to 'score' points on each other. The encounters require Harry to learn manners, bloodlines, heritage etc though he's not 'stuck up' like Draco- whom he doesn't like much. The occasions are actually useful for Sirius and Remus as they discover the 'grey houses' and even those among the DE families who don't want Riddle back; another useful benefit is when word gets back to the Blacks regarding who Sirius is consorting with he is re-instated on the family tree- obviously having grown out of his 'childhood obsessions' - Sirius does not bother to correct them, or visit, though he does spend their money.
Goyle also gets something for his relationship with Harry- concern that Harry might only be exposed to 'dark' families has Harry meeting the Tonks, Longbottom's, Wesley's, and the Lovegood's, and of course he takes his best friend along. Greg is not stupid either, exposure to Harry and a slightly happier, supportive home life has allowed his intellect to prosper, and with Sirius and Remus' help he is seeing more than the viewpoints of his parents, though he knows better to say such in their presence or his diary. Greg and Neville bond over their joint love of the outdoors and growing things, while Harry likes to teach them both to cook what they grow. Greg and Ron at first don't get on, until Greg manages to get a practical joke over on the twins which instantly wins Ron over. Greg's knowledge of Trolls also impresses Charlie and Percy. In the latter case Greg manages to crack the 'intellectual Wesley' out of his carefully constructed shell by inadvertently dangling interesting knowledge in front of him. By the time the boys are 11 Percy is a regular non-term time visitor to the Troll 'farm' - his enthusiastic letters ranting about Trolls in the Prophet actually causes a small spike in the use of security Trolls, even Hagrid 'trials' a couple to help stop the beasts from getting in from the forest.
Sirius and Remus are visited by the Order members from time to time' Dumbledore, McGonigal, Sinarta, Pomfrey, Hagrid, even Snape drops in. Harry likes Snape as he has stories about his mother, and his skill at cooking translates well to potions- though the Potions master won't let Harry know, he quite likes the boy as he is more Lily than James, despite having the Dog as a 'father'. Greg also gets to meet some of the collection- though he is wise enough to to mention having seen Snape at 'meetings' at his house to anyone but Harry.
When the boys go off to Hogwarts they are early on the train and save seats for Ron, and Neville. It is thus Hermione who drops in asking for a seat, and Greg who lets her in; a more knowledgeable group will help her get over her belief in authority quicker, esp. with their mixed bag of parents, brothers etc. Though Hermione's belief in books can't be shaken yet. When Draco and Crabbe drop in there is the usual sneering, but Draco does not have to try and become Harry's friend as there is already an 'agent' on the case so he just plain unpleasant instead.
At the sorting Greg is called first and he impresses the Hat with how much loyalty he has for Harry, but also how much he wants to stay out of the fight with the light and dark. The Hat sends him to Hufflepuff. Hermione wants to be with her new friends and joins the Badgers. When Harry is under, the Hat tries to influence him towards Gryffindor, but Harry wants to be with Greg and Hufflepuff gets a new member. Neville ends up in Hufflepuff too after the Hat decides the security of the Set will do him good. Ron ends up in Gryffindor as his desire not to let down his family is stronger than his loyalty to Harry and co.
The only difference from canon in lessons is Harry gets Snape's questions right, and the Greg-Harry team scores well on their first potion. Hermione and Neville do ok, but Snape docks points as Hermione did most of the prep. The Quad will work to resolve Neville's problems with potions. The other difference is Harry tells them something is odd about Quirrell, and they resolve to keep an eye on him. Harry does get to fly for the Hufflepuff team- and Snape saves him. Hermione does not have to set anyone on fire.
The Quad tries to stay friends with Ron, but he is jealous and unhappy about not being with them. Despite the Twins, and Percy trying to help Ron cuts them off. By Halloween Ron is outright hostile to them and causes Hermione to burst out crying. The ensuing brawl is broken up by Snape- who actually saw everything, and he takes Ron away for a talk with the Headmaster. The remaining Puffs rally and manage to persuade Hermione out of the bathroom. They goto the feast together and manage to have a good time- raising glasses to Harry's parents to honour their memory.
When Quirell busts in regarding the Troll, it is Greg who suggests they go looking for it as 'he's probably confused and hungry' - they stuff their pockets with food and race off. They find the Troll on the 2nd floor destroying a bathroom. Greg talks to him and realizes he's one of 'his' - the Troll, called Grog, calms down and explains that he was patrolling the grounds as employed by Hagrid, when someone cast a spell on him and he was all mad, then in the castle, and he is not sure why. The Quad feed the Troll the food they have, and Greg calls a House Elf for more - the Hogwarts Elves are wary about approaching a Troll, but they leave the food with Greg and retreat. Greg, and Harry keep Grog company while Hermione and Neville fetch the Staff. The rather astonished senior staff find Harry and Greg unharmed leading Grog back out of the Castle, feeding him food, and keeping him chatting. They are awarded some serious House points, and when Grog is examined evidence of spellwork is found, though they never find out whom's. Grog and his two brothers will become regulars at Hagrid tea evenings.
The 3rd floor mystery does not interest the Quad as they 'know' it's not Snape after careful questions are asked back home, and Harry all but asks the man himself what was going on and Snape tells Harry the staff have the situation under control, the Quad drop it.
At the end of the year Harry is hospitalized after a searing pain in his head, and his scar bleeds. Pomfrey keeps him in the wing, and his nervous friends visit him under Harry's invisibility cloak (Dumbledore returned it to see what Harry would do with it). Thus, the Quad are all there behind closed curtains when they hear Dumbledore bring in a deceased Quirrell, and a Snape complaining about Dumbledore's 'risky' choices with a cursed soul sucking Stone. "It worked though; the shade is captured" a weary Headmaster says. The Quad decide to keep this to themselves.
End of term comes, and the foursome sit together on the train planning summer activities, including a visit to the Muggle world to visit Hermione, and lots of worry-free party time!