You can do do down throw to fair and upair, then fast fall and use an uptilt and follow up with something else, a grab if they're low enough, or an upair or nair depending on how high they're knocked back. There's also down throw to RAR bair for the kill, but your timing has to be perfect, which also accounts for down throw to up b. With the improved hitbox it's much easier to land, but I'd only ever use this if they don't DI at all or di towards you. If you miss you'll most likely be punished hard so keep that in mind. Another mixup i like to try is at atound 80-110ish percent depending on the character, id use down throw to nair, and then double jump and use cyclone. This is very escapable, but people only seem to escape when they spotdodge. I've caught people with the very last hit of cyclone when they try to jump, and it still kills, which is pretty satisfying. When you go for a down throw, you could follow up with a jump-canceled up smash for the kill (first you'd have to condition your opponent to spotdodge, just by repeatedly using down throw combos), or what i like to do sometimes is throw out an empty short hop, and as they suffer spotdodge endlag I'd fastfall and then hit them with a forward smash, preferably angled up which would kill even sooner. Downside to this one in particular is if they DI correctly and they go out of range, or they actually throw out an attack like a nair. Another general thing i like to do is after a regular down throw combo of fair to fair or fair to upair, I'll throw out a cyclone. Also don't forget about down throw to dair near the ledge of the stage, or even at higher percentages on stage if you land it you could throw out a nair afterwards ff they miss their tech and end up rebounding of the stage.
I know you asked for downthrow combos but another thing I'd like to add is luigis up tilt combos. Usually when i read a roll id just uptilt and lock my opponent in continuous uptilts, although it doesn't work on opponents with frame 3 moves, so keep that in mind, after they start to get knocked out of range you could follow up with a nair, then at later percentages an upair, and even a bair if they're DI calls for it (fair too but you might as well stick with upair on this one just for the extra damage) in the case that you use nair when they're at a low percentage and they don't get knocked away from you, I'd just follow up with a cyclone immediately. Another thing to just throw out there is jab or double jab into up b. This is something I'd only try if im up a stock because it's not guaranteed and can be punished heavily. Also don't forget your jab into grab. I personally like to use double jab into grab, as it results in more damage, plus the dashing animation you have to perform in order to grab acts as a buffer so you don't have to time it as well, because if you don't time jab to grab well you'd just end up sending out your double jab and possibly your triple jab as well by accident.
Sorry for the long response, hope some of this helps
