Honestly, I think mentioning the 6* dressbloom was only to explain the mechanic and not as a hint to anything that will come.
If anywhere however, I think it would appear beside the golden dressblooms in the life crystal shop, which would force me to reevaluate my priorities.

Being what they are, ancient ampies will surely return as an exchange prize - I agree it only makes since since you can't max an anyone out with the currently available amount.
Will there be a farmable way of obtaining them? I think it makes sense for it to appear, however I can't see that happening anytime soon - the item is too fresh.
I haven't translated the plot of the battle with Nidhoggr for myself yet, so I can't say if they had any plot device behind them or if Nidhoggr was defeated once and for all, but I guess I can see similar events in the future with another giant ancient pest as the main focus.