Whilst Sailing is the obvious skill for an exploration focus, it has a number of challenges that are very, very difficult to overcome.

One is that the new areas to explore have to be new areas, rather obviously. I'm actually not too worried about this, since Jagex has done a decent job (IMHO) of expanding the OSRS map over the years, but it is worth noting.

The bigger concern to me is the actual sailing component. OSRS is very unique amongst MMOs with it's square-based tiles. I'm not sure how you develop any form of sailing with this where it doesn't feel very, very clunky. Can a boat turn, or does it require that it be trapped in it's own instance and the boat isn't turning but instead the map is "turning" around it? Does this mean pirate battles are actually just Castle Wars but on the Tempoross map with static ships?

By virtue of oceans being, well, open, you also have the question of how to segment how and where ships can go, and when. Do certain places just tell you to fuck off if you try and dock a boat there before doing certain quests? Basically, Sailing has to integrate into the exploration progression of the current game and doesn't have the mechanics to do so in place yet.

The Spirit Realm, by contrast, can avoid these problems quite effectively. Keeping the old school graphics feel with spirit world twists on the current world feels quite doable. The core exploration mechanic is walking around, same as OSRS- no complicated new design work needed there. And the tools used to limit player progression across the map in the real world can be much more easily transferred to the spirit world in some form or another- That gigantic wall around Meiyerditch will still be there, along with other gates and mountains and oceans and more.

Running around the Spirit World searching for the things that are different compared to the main world, finding new creatures/spirits/resources, neat stuff like that. And hey, maybe the Dark Altar in Zeah corrupts the spirit realm there, and Zeah's Spirit Realm is incredibly dangerous but more rewarding.

TL:DR - Shamanism Spirit Realm is best option for combining that exploration and discovery we want from new stuff with old-school feel because it can use the current map's exploration progression systems and graphics as a base to build from.