I only have 1 shiny Magikarp. I have several good offers for it, people are willing to give me legendaries. I didn't do raids until this very week, so no legendaries except Regice and the research task birds.
What would you consider worthy to trade for it, considering the fact that I might not ever find a shiny Magikarp again (also: what legendaries are most likely to come back, and are thus not as essential to trade for)?
Among the top-tier legendaries, Raikou and Entei are expected to be the next field research breakthrough rewards, while Groudon and Rayquaza are likely to return in the near future.
If getting an Ex-Pass is very difficult in your area, Mewtwo would be a good choice. Your post implies that you do not have Kyogre, which just left us and is unlikely to return soon, so another great option.
You guys each have 1,000,000 Stardust to make this trade? That's more impressive than either of the mons being traded.
I don't think a shiny karp is all that great. I mean, you're only getting it to satisfy some vanity. Because functionally it's no better than a typical karp. Moreover, it's likely to get its stats tanked in the trade.
