Sounds like a good idea. I could even see it as entertaining even without brining in the Negima! cast or events and not having them trapped there, but simply having hyjinx and advantures in the magical world before returning to the regular world and Nerima a bit later than cannon, producing different events in Nerima related to Ranma's presence. For instance Akane might still be having issues with the morning battles, or something might have happened regarding them during that time. He'd probably miss the fight with Kodachi. And Kuno might end up being right about Ranma being a 'foul sorcerer' by the time he arrives.
Ranma's initial arrival in Nerima he is probably still on the border between potential to start going ki use or magick use to boost his martial arts, and the magic world could either cause him to go the magic boosting route instead, or cause him to find out more about ki, or even try going the Kanaka route instead if he can find out lore about that and probably a teacher needed (which is probably quite rare, and what Ranma could find likely not so competent, as if I remember right it is implied that the only real master of Kanaka techniques left in both worlds is Takahata-sensei, but Ranma could still probably find a non-master who can use the basics and supply him with the pertinent lore and initial training, and it is a rare and difficult art to begin with). Having the Tendo reactions to Ranma either using more advanced ki arts like ki blasts and higher level shundo and powerful battle auras, and possibly some basic stuff derived from Shinmei-ryu and ninjutsu or using kanka or magic techniques when he first arrives, not to mention other martial artist reactions, and the reactions of the Amazons would be a sufficient plot twist to make an AU Ranmaverse crossover/divergence like that nice.
Just traveling in the magic world when the portals are being destroyed though... yeah, I could see Genma going the treasure hunter route, and taking Ranma along with him. The only problem is that Genma isn't as great with traps as he is with thieving, especially in comparison to the Library Exploration Club crew, who are used to high level and often magical traps (disguised as mundane ones). This could be a source of conflict and difficulty for them early on and would add to the story, especially if Ranma quickly outstrips Genma at dealing with traps and becomes a better treasure hunter.
Genma would also have trouble if he tried to sell Ranma off or racked up debts in the magic world, given the way slavery operates there, and likely would sell Ranma into temporary slavery pretty quickly, likely a short temporary one worth the cost of a meal or something, only for while Ranma is enslaved Genma to get himself stuck in a bigger contract he'd have to work off in the arenas as a gladiator, and Genma would likely have trouble dealing with the level of fighters past a certain point, as while powerful for the Ranmaverse, he isn't that big in the Negimaverse, which has a steeper power grade and a lot of smart magic user fighters who won't be as easily fooled by Genma's tricks, which would slow him down in earning his freedom, since he can't go to the biggest fights unless he trained a lot and got a lot better, and might teach him a bit of extra humility. I can also see repeated escape attempts resulting in getting zapped with punishment orbs for Genma. Ranma on the other hand would probably just work at a restaurant for a couple days to pay things off, then get an actual job there while saving up money to pay for a cure for his Jyuusenkyo curse and waiting for Genma to be able to pay off his own debts in the arenas, and convincing passing fighters and gladiators and bounty hunters and such to help his training. After Genma learns his lessons about being enslaved and is free again, that is when they'd turn into treasure hunters for a while, likely until they get acquainted with the Negima gang. They'd probably return to the old world by the time the current Negima arc is finished in their story, and then go to Nerima with the skills they gained in the magic world, probably later in the year or a year after their cannon arrival, unless Genma makes some deal in the magic world to replace the Tendo deal during the time he might think they are stuck there.
"Health, Learning & Virtue will ensure your happiness; they will give you a quiet conscience, private esteem & public honor." Thomas Jefferson (1743 - 1826)