We do not have enough Mega Drive tracks in here.
Of course Shining Force II the greatest game ever conceived has a ballin’ overworld theme, Wandering Warriors, which doubles as one of the earlier battle themes in outside environments.
The same composer Takenouchi Motoaki also composed the soundtrack of Landstalker and I guess Ryle’s theme / Nigel’s theme qualifies as the standard overworld theme of the game.
But the one I really want to highlight here is Phantasy Star III’s ground theme by Takeuchi Izuho. Like many aspects of the game (the generations system etc.), the soundtrack went for something pretty unique and innovative even if the result possibly did not match the intent. The game’s overworld BGM starts with a pretty simple (and mediocre) two channels-wide track, which probably left a poor impression on people who checked the game but didn’t go far into it. The secret behind this strange tune is that each additional party member adds a channel and instrument, progressively revealing the real overworld theme. Here’s a nice little mix that links the different variations in sequence. The second version of the theme kicks in at 0:49, the third version at 1:37, the fourth version at 2:26 and the complete version plays proper from 3:15. It’s a pretty neat idea!