I think it's going to start big. As you pointed out, they've already got a large player base for it, plus in articles & interviews I've read (don't ask me for links, look it up yourself), Jagex have stated that they've already got a decent amount of content in the game ready to satisfy everybody at launch. On the subject of levelling, I read somewhere that 'skilling' is going to be different from RuneScape. The developers at Jagex are trying to make progressing through levels less of a grind, & more of an activity that does not lose it's appeal as fast as, let's say, fletching bows/mining ores/cutting down trees.