Here's my take on the Wario timeline
>Wario was born a Star Child. However, for some reason his parents were unable to receive him and so the stork just dropped him where he wanted to be, next to loads of treasure. However, Baby Bowser eventually acquired that treasure and Wario was promptly sent to the Brooklyn region of the Metro Kingdom to be looked after by other humans
>Wario's adoptive parents were a stingy deadbeat involved in construction work (Foreman Spike) and his hardworking housewife. His mom was related to Mario's family and that's why they are cousins.
>Wario and Mario actually did used to be childhood friends until they had a falling out and Wario's family moved to Diamond City. Whatever happened between them hurt Wario a lot more than it did Mario since Wario is the one who brings it up and Mario only thinks Wario is crazy
>Wario's father left them and his mother eventually took up farming. Much of Wario's strength comes from toiling away in the fields to support his ma. His ma is still alive and well to this day and she is very proud of her son and his weird friends.
>Wario fit in a lot better in Diamond City and that's where he met a good portion of the WarioWare cast, like Jimmy, Dribble and Spitz, and Crygor. However, he never quite forgot his grudge with Mario, and when news of Mario's heroics across the world reached him, he was furious and made quite a few attempts to attack Mario, all of them unsuccessful
>When an alien spaceship crashed by his house, Wario met Orbulon, who promptly revealed his plans for world conquest and introduced Wario to his friend Tatanga. Thus, Wario's initial plan was formed.
>After the events of Mario Land 2, Wario decided that openly attacking Mario was getting him nowhere and figured that wreaking havoc on his allies and surroundings would work out better. Games like Wario Blast and Wario's Woods take place here.
>Off the heels of his first treasure hunt (Wario Land 1), Virtual Boy Wario Land happens while Wario is making his way back home.
>The events of both games went a long way to make Wario's self-confidence a lot stronger than it already was, which is why he stopped attacking Mario (at least for a while) and took up party invitations. He came to hate Mario considerably less and nowadays he mostly just antagonizes him to leech off fame and because it's fun.
>Wario Land 2 happens and Wario starts to build up quite the life for himself away from Diamond City, going full warlord and acquiring his own castle
>Wario Land 3 and Wario World happen within a very short gap, and both experiences shake up Wario quite a bit, as he loses his castle and riches, and encountering guys like Rudy and the Black Jewel make him realize his own mortality a bit. He becomes somewhat homeless during this period.
>It's during this time that he encounters Waluigi, another fellow mischief maker angry at the Mario Bros, and he becomes Waluigi's partner and training coach. Waluigi doesn't join him in his treasure hunts but they live together in the Mushroom Kingdom for a brief while.
>Shake It! happens and Wario once again goes on another treasure hunt only to lose it. Still, the events of Shake It and his fight against the Shake King give Wario his fighting spirit back
>Wario Land 4 happens and the experiences with Shokora and the Golden Diva make Wario change his ways a little while also leaving him with treasure for once
>It's at this point that Wario decides to return to Diamond City, a stronger, better, and smellier man that he was when he left. However, he has no idea what to do now.
>Master of Disguise happens, which gives him the idea to enter videogames as a market
>Wario calls his old friends and new ones, and decides to open up WarioWare Inc.
And the rest is history.