I don't know if you've been following the storyline but we're probably almost ready to introduce Sebastian...and then later Jim. The story so far has been kind of based on The Empty House but obviously the ending will change quite dramatically from that so it doesn't matter too much about sticking to it. Basically Mycroft is returning with a dummy which we'll set up in the window to look like me then Sebastian will shoot at it, John and I will try to stop him and fail miserably somehow...so Seb will basically end up getting away.
We don't know where he's gone and can't really do anything immediately so then I guess we just try to get on with our lives for a bit, and maybe we hear of a new case coming up and start to investigate it but perhaps what we don't realise immediately is that it's got something to do with Moriarty...and Jim maybe you could start playing with us like doing crimes and we don't realise it's you because I blatantly think you're dead...and then maybe bit by bit we piece together the clues...or maybe you just make a shock appearance or that old classic - kidnap John or really just anything you want at this stage...it's over to you...but basically we'll need to come to realise you're not in fact dead.
Also, in the original stories, Seb wants to kill Sherlock because he's angry at him for killing Jim. So maybe our Seb can believe that Jim is dead too...then you guys can have a happy reunion like John and I did and it'll give you chance to chuck in some shameless MorMor.
Let me know what you think of the ideas and whether you have any other suggestions??