The following is just a quick tutorial on making your own D&D sandbox. What's presented her should give you the bare bones to jumpstart a long-term campaign.
A. Starting location such as a city, village, settlement, etc. Wherever the PCs come from.
B. Handful of small locations of interest - temples, dungeons, tombs, towers, castles, swamp, forest, valley, hills, etc.
C. The Big Bad location, this is where the PCs aspire to go... eventually.
A. Good guys vs bad guys.
B. Shades of gray (no one is all good nor all evil).
C. Directly relate to the PCs' background and/or goals.
III. What's The Deal?
A. Rumors, myths, and legends.
B. The immediate micro-hook that initially draws the PCs outside their comfort zone.
C. The far off, long-term goal or macro-hook that can be pursued when the PCs are ready.
Ok, that's it. If you want to hear me talk about it, here's the link to my YouTube video.