First Question:
Since the Arduino Giga r1 wifi is 3.3 volts...
Doesn't that limit the shield/daughter board choices?
Yes, there are 3.3v to 5v step-up chips/boards but wouldn't it just be easier to just go with a 5volt board and have all the historical shields/daughter boards at your disposal?
Next Question: What is the most powerful 5 volt board out there?
Well, yes; but that's nothing new - Most "recent" Arduinos are 3V
As @anon57585045 said, that depends on what you mean by "powerful"?
Also, what you mean by "out there" - is that looking just at Official Arduino boards? Or at any Arduino-Compatible boards? Or just any boards at all? Or what?
Does anyone make a 3V <--> 5V "interposer" shield?
ie, you could plug it onto a 3V Arduino, and it would convert all logic levels to 5V so that you could then plug a 5V shield on top of it.
Bonus: if it could autodetect (or be configurable) to go the other way.
If not, maybe it's a product opportunity for someone ...