I happen to be in the minority that thinks Big was a worthwhile addition to the SA1 cast. His gameplay is much slower, but I found that to be a refreshing change of pace, occasionally stopping in the middle of other character stories to just sit down and think about the story so far and how the character stories overlap while reeling in some fish.
As far as SRB2 is concerned though, there's not really a way to make this sort of thing work. Big could make for a funny cameo SA2 style, but that's about it. If he was to be implemented as a proper character, he would need significant overhauls compared to SA1, and a complete departure from the fishing gameplay. What makes this not worth doing is that he's such a physically large character, he would need unique pathways off the main path from time to time due to his physical inability to get under tight spaces or through narrow enough hallways/doors. Keeping him as at best a cameo if including him at all is probably the better option.
If they were to ever implement a 7th character, they should serve one or both of the following purposes:
1. Acting as a character to put Sonic's thok onto so that Sonic can be given his new ability. Preferably, this character should also have other things making them unique, other than just being "Old Sonic but with a skin".
2. Having an interesting new mobility option that allows them to traverse through the existing stages in a way that's different from the other characters, but still fun and balanced.