I have a few but none of them are guaranteed. At 0% vs fast fallers (fox is best), you might be able to catch someone off a missed tech from forward throw. Once you forward throw, dash to shield and then forward tilt two times to reset them. After the reset, it seems like an optimal punish would be down throw --> side b --> jump fair. I believe it was pogo shark who came up with this combo. I've landed it several times however, most of the time people will tech it. I only use this vs fox and captain falcon.
Here's one I like to use more though it can be a tad unsafe so use it when you know it will land. Side b flip kick is harder to tech than forward throw from what I can tell. At low percentages, after landing a flip kick you can forward tilt twice if the opponent misses a tech to reset them. From here you can down throw into two back airs and regrab which isn't true but often works. Or you can down throw --> side b --> fair.
When up throwing at 70+ %, opponents may be inclined to air dodge, especially if you performed a throw on a platform. Knowing this, you can read an air dodge, land a slightly delayed down air, fast fall to aerial if they miss their tech. If you know your opponent likes to mash jump or attack out of throws, up throw to down air them at low percentages into aerial follow ups or read their techs to regrab them.
