It feels like the Red Lotus won't play any part in the last 3 episodes of the Avatar Franchise. Lord Zuko mentioned that there are other Red Lotus members out there, so I feel that the Red Lotus will be one big loose end when the show ends, unless they plan to move the Red Lotus over to the comics...
Our only hope is that the Red Lotus somehow drops in unexpectedly during when Kuvira attacks Republic City. And we don't know if Xai Bau is alive or not, or if someone else is leading the Red Lotus in his place now that Zaheer is incarcerated, but in any case, it doesn't feel like Red Lotus will come into play before the show ends, but I could be wrong since we can't make judgements on upcoming episodes based on their titles. For example, "Day of the Colossus" could mean anything, even Zaheer merging with Vaatu like in Varrick's mover idea from "Rememberances" and becoming huge like Unavaatu, which could possibly make for tying up this particular loose end.
I feel like the writers are underusing their potential if they are said to still be a threat. The Red Lotus should be used as recurring enemies not related to the plot, just like how AIM and HyDRA are used as recurring enemies even if they aren't the main villains.
Unless they were to be used in a future series like the avatar after korra (or a continuance of korra later in life) where their objectives would come to fruition - thus being a threat in the next series - I don't think them having any form of appearance would really be relevant.
There is no reason to tie all loose ends. One of the good things about the first Star Wars trilogy is that it didn't. Knowing that the Red Lotus still lives to fight another day gives the world a sense of scale and context.
I can't say as I'm really bothered. The three most evocative members of the Red Lotus are dead; the closest thing they had to a strategist is chained up in an oubliette. And considering what's already going to need to be wrapped up in the last hour of the show, throwing in new characters at the last minute would just detract from the plot.