Mass Effect - Replay values of rpgs

One of my favorite ever game was Mass Effect: great story, great graphics, epic choices and all sci-fi stuffs.. aliens, weapons, ships and so on.

In these days I'm playing Mass Effect 2: nice start, same point of Mass Effect, but with a different angle, and many changes.

Following my feeds, I've found today a little article about Mass Effect 1&2, centered on replay values of these games.

Many times on net I've found articles where it's clear that few peoples finish games todays, so why build great and re-playable games?

My thoughts on this topic is that as developer/designer in video games you must find first a reason to justify another ride on your games and as second point make second run more fun and more interesting for players. This is difficult part and many, many successful games (from indie to AAA title) offer single player experience as one-run only. If you play games another times, nothing changes.

So why add replay values to a games? One reason is because video game genre require it: rpg is a good example, but also fighting games is interesting on this topic.

I think point is always the same, like people in Blizzard says: "Easy to play, hard to master".

I'm one-man developer, so replay values it's over my possibilities. What can I do ? How can i offer with same type of gameplay new experience?

I can move from good design in levels (see Zombie Employee, best example of my work in this area for now), to random generated levels.

Another ideas ?