In my opinion, Oddworld has always been a bit of a niche game, very appealing to a relatively select audience. If you try to make it more mainstream, then a lot of it's appeal is at risk.
It seems to me like having collectables and multiple difficulties would be nothing more than gimmicks to encourage gamers to replay the game once complete. Of course I cannot speak for everyone, but I imagine that the type of person who plays Oddworld values story, atmosphere and character development as much as gameplay- you know, the gamers who don't skip the cutscenes
What I'm trying to say is that I do not need any gimmicky incentive to replay Abe's Oddysee. I replay it every so often because I enjoy the story, characters and environments, etc. I do not replay purely to get the last trophy, that last collectable or to 100% the game by completing it on 'hard'. Abe's Oddysee simply does not need these features to get it's fans to play it again!
I would be very disappointed if such an artistic game were reworked to appeal to the short attention spans of the fps generation. Of course trophies/achievements wouldn't detract from the game- I just think that Oddworld has always been about story and shouldn't become just another 'Hey, that Odd game has thirty trophies! I don't care about the story, I just want more trophies' kinda thing
