Should have the game start with a new protagonist. I think the new sam is different enough to still work as a new guy, just give him a different name. Have Michael Ironside Sam mentor him and put him in an advisory role. Would have been awesome. I think the approach they're using now is kind of insulting towards Ironside. If they kept him around as an older advisor/mentor, it would have been much more respectful, I think.
Don't read the official site or the fan wiki but they sorta have a reason to put Sam Fisher back in play.
I don't know if replacing Sam Fisher as the playable character would do the franchise any favours because in all honesty the franchise is pretty much Sam Fisher. If anything Splinter Cell is much more character focused than any other Tom Clancy franchise. Pardon me for not knowing too much about the old Rainbow Six novels, but if you look simply at the games, I really cannot name any other TC protagonists except Sam Fisher. Nor are any of them memorable.
I still have a few reservations about not having Ironside do the voice, but,
1) Eric Johnson (the new guy) has worked with MI before
2) MI has pretty much trained Johnson on how to play Fisher, so while it won't be Ironside's voice, it'll have his demeanor and tone, etc. I assume he still had/has input on where the character of Sam Fisher is going, i.e. consults for them rather than doing VO work since Sam Fisher is pretty much his baby (who had no personality of any sort before MI took the reins on Sam Fisher's character development since game 1)
3) Johnson pretty much has come out saying he's going to do just that and not try to put his own spin on it because of the history of Sam Fisher.
4) From what they are shooting for the cutscenes where Sam Fisher has to do tacticool things, it seems like a bunch of work. They had a stunt director (who could probably effortlessly kill everyone with the training he has) teaching Johnson how to do them and he's pretty much been practicing all the time when off or between takes that he makes it seem natural.
Also when people got flown into Toronto to play the game they had realized at the end of the day they didn't notice Michael Ironside's voice had been absent.
Though honestly since the other languages are getting dubs with their respective voice actors, it won't line up with lip syncing or anything in the English production, so really casting was really just for cutscenes and there really isn't nothing stopping them from having Ironside record the lines (for DLC $$$$$$$).
Then again they haven't really sold it for anyone who hasn't spent much time listening to the difference to care other than kicking off a fad of stealth games giving their long-time VOs the boot. Either way I think keeping Sam Fisher ageless/immortal/whatever and as the playable character is the better choice for the franchise (unless it bombas) than replacing him with someone else (because everyone would just bitch about it anyway).