This is going to be very long, because I've thought about it in great detail, apologies...
It would be an RPG/third-person shooter based on the manga/anime Black Lagoon. For those unfamiliar with the series, it's about a team of modern pirates/mercenaries (the Black Lagoon Company) in the south China seas. They have their own ship, but much of the action takes place on land as well. The main setting in the game is Roanapur, a city that's simply riddled with organized crime, where everyone is out for themselves, violence is everywhere, the few police are almost all corrupt, and no one can truly be trusted. It's basically an entire city of criminals. Sort of GTA-esque but seedier.
It would be party-based, where one could switch out the player-controlled character in different circumstances. Each member has their own strengths and weaknesses, and you could spend XP to increase each member's stats in a particular area. The four members are:
1. Revy, the gunslinger, the muscle of the group. Sections where you primarily use Revy would be shooter-based, closest thing to it would be like Max Payne, with bullet-time and plenty of acrobatics. Spending points on Revy increases damage done in gunfights, and also increases intimidation options in dialogues. Also, she's in good with a particular faction that no one else on the team is, and can pick up tips and help from them. Oh, and she also gives skills for ship combat that can't be obtained otherwise.
2. Rock, the diplomat/strategist of the group. He would be the main person to navigate conversation-based gameplay (though Dutch is decent at it depending on who you're speaking to). If you fail conversation choices, all hell breaks loose and fighting starts, so you'd switch to Revy. (Rock won't even fire a gun, he's a total pacifist). Spending points on Rock increases diplomacy ability, opens up new options to complete missions, big reputation boosts with all factions, and also gives a significant boost to Revy's abilities (she really likes him and performs better when he's around). Rock also gives a massive bonus on any missions where hostages/kidnaps are involved (he is Stockholm Syndrome incarnate).
3. Dutch, the leader of the group, he oversees and collects their contracts, in addition to owning and piloting their ship. He's also pretty good in a fight, though nowhere near Revy. Spending points on Dutch increases a number of things--his own damage in fights, the handling of the ship (for ship combat purposes), and he's the biggest contributor to the team's overall reputation bonus (which affects how much money you get from contracts and the number of them available). He has a lot of connections with many underground factions. As the leader, he gives a moderate bonus to everyone's proficiencies.
4. Benny, the hacker/intel guy, and also the team's driver (Dutch can also fill in at this). Spending points on Benny also increases a number of things: Your ship's stealth rating (how easily you can ambush or evade ships at sea), buffs/debuffs on ship combat, hacking options/extra intel on certain missions, and car handling/speed for car chases. Benny also can make the team some extra money through hacking side-missions that only he can complete. Like Rock, he is useless in gunfights, but, unlike Rock, he can be upgraded to at least contribute in them.
So the game plays out with the team acquiring different missions/contracts, fulfilling those contracts, and making XP and money which goes into upgrades for the ship, car, or guns for the team. It would be divided between A) Third-person gunplay, B) Negotiation/conversation, and C) Open seas ship combat/smuggling/pirating. With a side-element/minigame of hacking. The overall goal of the game is to acquire enough resources and upgrade the group's overall prowess to be able to take on the villain of the game, some shadowy organization that has, for some unknown reason, started targeting the Black Lagoon Company and their connections.
The most important aspect of the game is that there are multiple ways to complete missions depending on how you spend your points and money. If you want to smooth-talk your way through it with very little fighting, you would dump most of your points into Rock, with Dutch/Benny as secondary. If you want to kill every last fucker who gets in your way, you'd go Revy with some Dutch/Rock. If you want to create your own unassailable pirate empire, become so big that no one could touch you, you'd go Dutch with Rock/Benny.
Also, if you've never watched Black Lagoon, go watch it. It's violent and crazy and boisterous and awesome. It would make an incredible videogame, too.