What are the best non-alcoholic beverages for mocktails that still taste like a “real” drink?

Look for layered flavor (bitter + bright + botanical), built-in balance, and a finish that holds up over ice.

Do I need mixers, or can I use these as the base?
How do I keep a mocktail from tasting too sweet?
What’s the easiest “go-to” build for a group?
What if I usually like cocktails with more bite?