Mabinogi announced they're in the process of developing an engine update. A quick excerpt from the article introducing the Eternity Project by Min Kyunghun:

That means there's a lot of work being done right now--too much to give a meaningful estimate of the actual progress we've made. What I CAN tell you is that we're either working on the following tasks now, or will be soon:

• We’re planning to redesign our server structure and logic to improve stability.

• We need to not only recreate all of Mabinogi's existing content in the new engine, but also ensure it all works as you've come to expect.

• As we do that, we'll be revising the game’s data and scripts, improving them as we convert them over to the new system.

• Every map and instance and the countless components that go into building them must be improved in quality or faithfully recreated in the course of migrating them to the new environment.

• In the asset migration process, we may discover that some original pieces of artwork or design elements we need have been lost to time...which is understandable, given the many years the game has been in operation. While we work on redrawing and recreating any such material, we'll need to write up new design documents for content that needs attention and polish to bring it up to modern standards.

• Beyond these specific things, there are many other areas that will need work as we bring the whole of Mabinogi into this new environment.

There's several articles you can read up on the site. It appears they're going for a complete graphical overhaul using Unreal Engine (no indication of if that's UE4 or 5). Either way, exciting stuff!