A handy hint to remember is main scenario quests are designed to be a continuous story, in other words, the location where you complete a main scenario quest, the npc that has the next quest in the story will either be the exact same npc or someone in that immediate area. Accordingly, if you forget where you left off, look in the Journal under completed quests and Main Scenario quests, and take note of the quest summary for the last quest you completed. It will almost always mention the npc which that quest ended with and their general location.

If you're still having trouble finding the npc, this page can be helpful (in your journal take note of the title of the most recent main scenario quest you finished, and then check it's listing on the above page - the next quest it unlocks you can find through a link on the page for each quest.