So i got two ideas here that i want to talk about.
Reaper's Cursed Chest
This idea is about a chest version of the Reaper's mark flag, except player's can't turn it off. Once you have unearthed the chest, it will show up on the map for everyone to see. The chest rewards players with a large amount of gold and thus could serve as a lesser version of maybe a fort chest. The main point is that this chest is meant to tell other players "behold, i have a rare and valuable chest on my ship. Try an get it." Thus, unlike the Reaper's Mark, a crew of player's can then attack another without ending up with no loot for their struggles. True, a player could just drop the chest because of this, but again, it rewards a large amount of gold and other players can easly find it with the map.
Monster Battles
This is another idea i want to talk about, many people have complained about not getting any rewards for defeating the Kraken or the Megalodon. Well this idea would serve to fix that (plus i also wanna add in this other idea made by @Trickrtreat01: https://www.seaofthieves.com/forum/topic/74404/how-about-some-rare-drops-from-ghost-ships-forts-etc)
Basicly, you all know about the Skeleton fort and ship clouds, well in my idea there would also be a new one. A cloud based on the Kraken skulls we have seen all over the Sea of Thieves:
When you sail to these Kraken clouds, you would find a collection of shipwrecks surrounding the area, but... don't try an swim in the water to collect the loot... It would not be safe while the cloud is still over the area:
Now, i would also make it clear that when you get to such an area, the monster won't show up... until you play the right tune to awaken the beast:
Once you have done that, the monster will rise from the depths and attack you. If you defeat the beast, the loot from all the shipwrecks will float up to the surface, the skull will vanish and the water will then be safe for you to pick the loot up in peace.
Now some may be asking why i included a mention to Trickrtreat01's idea in this. Well it's because i thought that these rare cosmetic chests could also be a part of these fights, as another reward for defeating the monsters.
Also, there would be more monsters than just the Kraken and Megalodon for these fights in order to make things more interesting.
Like for example, this Crustacean Kraken from the artbook