...and the opportunity to open up markets in regions （that we don't currently sell a lot to） is a great one.

括号里为定语从句，关系代词指代regions，并在从句中作介词to的宾语，所以不能用where。

句子的主干为：the opprtunity is a great one.

to open up markets in regions （that we don't currently sell a lot to)为主语的定语。