Did a quick search and I didn't see that this has been discussed before, but I just ran across an interesting old article elsewhere on CBR:
After all these years, I actually hadn't heard this before. In summary, this was the original idea:
1. Mr. Sinister was supposed to be the creation of a mutant child who was unable to mature beyond childhood and who was raised in the orphanage with Scott Summers.
2. On a side note, Gambit was originally supposed to be a villain who worked with Sinister and was trying to subvert the X-Men from within (I suppose this was a little obvious).
So, which version of these characters would you prefer the writers to have developed?