What is there to do in the endgame? I just finished the game (kind of short really, good length but expected it to be longer) and there are still plenty of locations and vehicles that are left unexploited.

The go carts track being one of them.

The only thing i got when finishing the game was an e-mail about a statue for sale but no way point to it.

Apparently the two statues are at one two of the blue swirly things that cover the map. Finding the two statues and updating your profile with them on the rockstar social club will grant you 2 new missions.

The location of the statues is random though.

Yea, the Guardian Lion statues grant you two new missions in the game, but there are other things to do as well. There are random encounters like in GTAIV, where you can get missions from random strangers on the street, as well as continuing to buy/sell drugs etc. The game also feature Rampages throughout the world and the usual side-missions like driving a taxi around and other similar crap. You can also give the MP a shot, but it does seem rather limited.

No real point in continuing dealing drug really, i made enough money in one game week to buy all the save houses and to fully stock up on weapons (heroin is my friend). Did quite a lot of the side missions already, the porn star one is hilarious.

Heard of the taxi missions, if you complete them all all the taxis will become bullet proof but since i have already beaten the game there is no real point to that.

I think I am pretty much done with it.