Should bioware do one final shepard game?

You're speculating and I simply disagree with your estimates.  I think putting Shepard into a mediocre game would ultimately drive more people away from the franchise than it would encourage people to come back.  The moment word got around that Shepard isn't the "same' as he/she was in the OT and that gets spun instantly into "Shepard isn't as good as he/she was in the OT"; the sales potential of such a game just dies.  If they bring Shepard back without "fixing" ME3, you also piss off those fans who want him back expressly as a vehicle to convince Bioware to fix ME3 in the very specific way they want it fixed.  In the meantime, you've also driven away the people who have been saying they don't want Shepard back AND the people who are merely  looking forward to some continuation and eventual closure on the ME:A story.  You're risking the entire fanbase on one character... one for whom the fanbase has extremely high and essentially unrealistic expectations about.  Of the separate "factions" of this fractured fanbase, the easiest ones to please are the ones who merely want a continuation of Andromeda.  There is essentially no expectation that such a continuation would give us the exact same personality of Ryder or any of the other characters in the game.  The plot of the story could go anywhere.  Nothing really has to be retconned beyond what has already been done and IS already canon within the MEU.

I'm speculating that bringing Shepard back would most likely do more harm than good.  Since we're talking about speculation and future events, neither one can say for sure what will happen... but my crystal ball is every bit as valid as yours.  All we can do is agree to disagree and wait to see what Bioware actually does and then wait to see how whatever they do do is received by the game playing public (note - I deliberately did not say "fans" here because the majority of the game playing public 5 years from now are likely not the current "fans" of Mass Effect).