First, welcome to the forums.
Second, I'm probably too late because it sounds like the run was supposed to be for this last weekend.
Third, oh well. Let's talk a bit anyway.
I think your overall plot is solid. Having options about which faction to align with is fun and provides some agency to the players.
I ran a 3-year ganger campaign in the Puyallup Barrens, so a lot of what you're talking about sounds familiar. For the first half of the campaign, the most insidious threat - the one that the PCs (gangers) couldn't really defeat - was gentrification.
The Mafia and Yakuza are good elements to include, but remember that Puyallup is the lowest-of-the-low. The Kenran-kai are all but outcasts, and the Gianellis are the remains of the losing faction after some Mob assassinations / power struggles. While these are the runts of the litter, that actually makes them more interesting. Because they have so little to work with, thy both have this "beggars can't be choosers" mentality that makes them more open and more progressive that their more conservative counterparts. They'll recruit metahumans, they'll work with magicians, they'll go with whatever works. But to me that also means that they're probably not doing backroom deals with Mitsuhama to build facilities in the area. The Kenran-kai just aren't high enough on the ladder to negotiate something like that.
If you want to pursue the corp investment angle, then the first step is to figure out what part of Puyallup you're dealing with. You have Loveland (red-light district) in the west, Carbonado (mining) in the east, Tarislar (elven district) in the south, Hell's Kitchen (lavaflows) in the southeast, etc.
Loveland is a fun backdrop but it doesn't make as much sense to me for corporate investment, so I'd leave that alone.
Carbonado has mining, so you could basically take your pick of which megacorp wants to extract resources there. Mitsuhama would work, if you wanted to keep that element. You could also have them opposed by TerraFirst! eco-terrorists. Another interesting option would be Universal Omnitech, which is headquartered in Salish-Shidhe. Conveniently, that's just a stone's throw from Carbonado. (UO is headquartered in Vancouver, but Carbonado is on the Seattle-Salish border.) Federated Boeing might be another local player that's interested; they'd probably be familiar with the area from their extensive holdings in Auburn just to the north.
If you're down near Hell's Kitchen, you could have corps pursuing geothermal power plants. Seattle 2072 mentions that some of these were approved before the Crash of 2029 wiped them out. Maybe some corps want to dust off some of those plans and try again. I think Gaeatronics would make an interesting player here, since they already supply much of the power to Seattle. Maybe they're trying to consolidate that control, or maybe one of their competitors (Shiawase Atomics and Aztechnology would probably be the best fit) is trying to undermine Gaeatronics' control of the Seattle power grid.
Alternatively, you could do something agricultural. Maybe someone is making a move on Petrowski Farms, or is moving in next door and might not be a good neighbor. Natural Vat (subsidiary of Aztechnology) would be a good fit. NatVat took a big hit from Sirrurg in August 2074 when he wiped out the NatVat facility on Borinquen (formerly Puerto Rico). Maybe they're looking to expand into Seattle to diversify their holdings geographically. This could involve a lot of local investment, but maybe it involves kicking a lot of locals off their land too.
Don't worry about the briefcase. Leave it as a McGuffin. If the players open it, have it be something inscrutable. It's a message that means something to someone, but not the players.
Anyway, that's what I can come up with off the top of my head. If the session already went down, let us know how it all played out. I'll be interested to hear.
