Falcon's down throw can combo into nair, uair, or even knee. Down throw to knee can actually be pulled off as a true combo but only at certain percents on certain characters with certain Vectoring/DI. Down throw into uair into knee is another situational but true combo.
I see a lot of Diddy throw combos in tournaments. Mostly down throw into uair into fair or airdodge traps.
Mii Brawler has down throw combos into utit and most aerials. I've seen down throw into uair into upb kill but I haven't tested it myself so I don't know if that's a true combo.
Zero Suit Samus can combo uair off down throw at like almost any percent. Also she can even death combo Dthrow -> uair -> DJ uair -> UpB but it's very situational.
Sheik's down throw is as infamous as ever, to the point where people have found a pseudo chain grab with it oh boy.
Fox down throw I'm not sure if it actually combos into anything but at low percents it puts the opponent in a bad spot to get easily followed up on. I think if it get's DI'd inwards you can combo into dash attack or fair.
Game and Watch's down throw combos into Utilt, Nair, and UpB. Sometimes side B is possible I think.
Ike can combo down throw into nair at low percents.
I think Mac can combo down throw into grounded up b since it jumps so high, but I can't remember if it's a true combo.
Lucina/Marth can combo up throw into uair but it seems really situtational.
Pikachu probably has a whole bunch of follow up from his down throw but I haven't played with him much after the demo.
Ganondorf's down throw combos into Down B 2 (Wizard's Dropkick). Another reason why we need custom moves.