I picture MapleStory's world as a cylinder we walk around the surface of. Portals and towns link the surface together, and hidden streets are the doors into and out of it.
Hidden streets also spread players across training areas. Ellinia is a good example of a dense stretch of the cylinder, with layer after layer of depth to explore. Kerning City and base-game Perion have a thin shell with very few hidden streets underneath.
If Nexon wants to expand the world, new surface areas should come with depth added to the thin regions. The areas around Lith Harbour are a good candidate. Heavily used maps like the Damp Forest are another, since extra hidden streets there could relieve crowding at the level ranges that pile into them.
Depth only helps if the maps are worth using, so the new hidden streets would need solid spawn density and good layouts. Placement should follow where players currently compete for maps.
They could theoretically even use maple worlds and hold contests for user designed hidden streets. That way it alleviates some of the manpower they are apparently short on to design some fun new areas for the community and will revive some of the original plan with Maplestory Worlds.