OmniOutliner is the heir to MORE 3.1. Arguably it's passed MORE's outliner capabilities , especially now that version 3.5's LinkBack capability allows them to embed OMNI Graffle items in an Outline. The update is free for registered 3.x users.
OO is also a reasonable way to produce HTML documents. It can even create HTML with dynamic outliner capabilities. (Whey has no-one done a browser that can represents a structured HTML document as an outline? Odd.) Alas, it doesn't handle hyperlinks, a feature I've requested.
One of the reasons I love OO is the file format. Rename the file to a gzip extension. Open that. Rename Contents.xml to Contents.xml.gzip. Open that. Now view Contents.xml in a text editor. Plain XML. If OO went way, it would be relatively trivial to write a utility to translate the files to RTF or another format.
 MORE 3.1 also had a vector drawing capability, a presentation module, and a module for creating org charts and the like. So a true replacement would be OO + OmniGraffle + Keynote. OO is now integrated with OmniGraffle, but Keynote integration is limited to saving an outline in Keynote format. Nonetheless, that level of integration does make me more interested in purchasing Pages/Keynote.