Yes. There were a few games, many years ago, that used a 2D side scroller viewpoint, which were very immersive and highly explorable:
- Colorado / Starblade (same engine, action / adventure)
- Drakkhen (action / adventure)
- Sapiens (action / adventure)
- Stronghold (a strategy RPG)
You can find videos for all of these on youtube.
These all allowed movement "in" and "out" ("north" and "south") of the screen as well as left and right ("east"/"west"), between screens -- enabling a non-linear world.
The main limitation for the sidescroller is the restriction in vertical space. This was overcome in games like Golden Axe and D&D: Shadows Over Mystara by giving the characters some vertical movement room without changing the perspective too much -- this makes combat more interesting, and you can evade enemies and go past them into other locations.
Sapiens was a particularly innovative approach to game design considering how old it is. One aspect was that you could see your x, y position on a map, although each location was a side-scroller view. Stronghold does much the same.
Stronghold is a particularly interesting/unique game in that it used a primitive vector based approach to show vistas of the world behind the current location, however the perspective is much the same as a platform game.
Finally, I'd say that in some ways, there's even more potential to achieve greater world depth in sidescrollers than in top-down or isometric views because, like FPSes, this is a natural view for a human being to see other creatures, and their world from -- eyes level to the horizon. So it can actually be quite evocative as a result.
