Bring back good ol' cheat codes, that's what I say.
I loved a good cheat code. Infinitely improved the longevity of a game. And a survey I held at my college a few years back found that the variety of cheat codes in a game really factored in the chance of someone buying it. Because it'd give them shit to do when they didn't want to play the game properly.
Take GTA vice city, san andreas, III, II, I. Cheats galore! They were so glorious and fun to play with. I wasted hours just messing around with the cheats!
Sadly GTA 4 threw it all out the window, with a narrow selection of cheats, that were only temporary at best, and you couldn't access them when you wanted because of that stupid phone mechanic. I,e you want the restore health cheat to be available in the heat of battle, right after you've lured 10,000 police officers into your rocket launchers blast radius. But no, we got no such treat. The cheat is unusable while under fire... Now we have to resort to dodgy native trainers to edit the games values to cheat. Sure trainers are awesome, especially the GTA4 one, with many, many awesome functions, but It makes me sad.
I maintain what I have always said. In a singleplayer game, let the player fuck shit up, and go off the beaten track. In Multiplayer, you play by the freaking rules.