Modding GTA Online exclusive DLC content into single player is one thing. Doing it even better than the official version? Now that's skill.
The modding community has made it an unofficial mission to get as much official Rockstar content from Online over into single player as possible. Many players who like story mode as much as Online or even more, are a tad sore that Rockstar is only ever giving the multiplayer mode any love. While it is impossible to implement certain elements of the Online DLC in Single Player, things such as cars are entirely possible.
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You know what GTA V now has that Online doesn't? Drivable yachts. Granted, it's the yacht from GTA IV, the windows don't break, the interior is sort of empty and it bugs out often but you can drive it.
Modder Tall70 has ported over the model of the yacht from the previous numbered installment of the franchise to GTA V in it's (almost) full glory. The ship is a tad slow coming in with a max speed of 35 mph, but if you're cruising in a luxury yacht speed shouldn't be your priority.
While the interior is bare at the moment, as mentioned earlier the author intends to redecorate in the next update. He also intends to get most of the bugs fixed. Compared to a first release, the mod is pretty solid already. The color scheme can be customized and the rain does not fall through the roof. The yacht has proper collision, so you won't be clipping into the landscape or falling though the floor.
In fact, the collision is better than it was on the original version back in GTA 4. Now you can actually land a helicopter on the pad and have it stay there, though it's hardly stable.
What other GTA IV vehicles would you like to see make a return?