From what I understand of these characters, such a merger would be a bigger change than it appears at first glance. Death is a cosmic entity that originates with the universe itself. When it appears before Thanos, it chooses to manifest itself as a woman. In that guise, it is referred to as Mistress Death. Death cannot die, since it's not alive in the first place. It normally dwells in another dimension called the Realm of Death. Hela, as you pointed out, is a Jotunn, a daughter of Loki, and ruler of Hel(-heim) as well as Nifflelheim, two of the Nine Realms of Yggdrasil. (I believe all of Yggdrasil is considered one dimension). Hela is long-lived, but ultimately mortal. She deals in the business of dead Midgardian souls, and thus could be considered an agent of Death, but it's not like they have a formal agreement. Hela also would like to have control over Asgardian souls, which drives various plots. But dead Asgardians normally go to Valhalla, an area of Asgard overseen directly by Odin. Warriors who die with honor also go to Valhalla (in the myth they also have to be chosen by Odin. Otherwise, they go to Folkvangr, which is ruled by Freyja in the realm Vanaheim). Death and Hela are two separate entities. To merge them would be a big shift in the cosmology as currently established, both by myth and by Marvel (what I've summarized above is Marvel's telling; myth does not have a separate Death entity, but I believe Marvel's characterization of Hela is consistent with the being Hel of myth.) If they were to merge them, then it seems they would have to do away with Hela altogether, and have Death preside over Niffleheim and Hel directly. As Death is from the Realm of Death, which again is a separate dimension, this strikes me as a little odd. But maybe they can make it work. Having an entity called Death and a ruler of the underworld(s) who is separate would be confusing to the general audience, I think. That would be the main reason for merging the characters, in my opinion. But that might make for a bigger role than Krige suggested she would have.