There may be a possible fourth to the trilogy. This is speculation only.
Actraiser (the first game at least)
It uses some of the same systems (health, fonts, etc.) and deals with regeneration of a world. Doesn't fit in exactly with SB, T or IoG but, ya never know with older RPGs. Plus, it was hard as hell to boot at the end.
Level ups with population growth? ugh!
In the time it has taken you to read this quote, I could have hacked your system!
Well, if ActRaiser was anything, it would be a prequel. In a sense, Soul Blazer was a pseudo-sequel to ActRaiser, but the connections are even more loose than the connections between Terranigma and Granstream Saga, the latter of which was meant to be the fourth game.