Fireplace insert or a whole new fireplace
If your firebox and chimney are sound and you mostly want more heat, an insert or a retrofit device is usually the smaller job, because it works with the fireplace you already have. A new build makes more sense when there's no fireplace yet, when you want a different look, or when the old one isn't worth saving. Cracked brick or a loose chimney often needs chimney repair before anything new goes in, and I'll tell you plainly which way I'd lean.

