I've just begun work on a sequel pack. It will feature 100 levels, again over 5 difficulty rankings, and this time, no repeats, whether from the original or from other levels in the sequel. However, that doesn't mean there aren't levels based on similar ideas (for example, one of the levels I've made so far has the same basic premise as No Time To Die - 20 lemmings, 100%, RR99, 2 minutes, 9 of each skill, and relies heavily on multitasking, but the solution and design are both very different than NTTD - and in my opinion, it's a LOT harder).
My aim is for the first rank (Cheesy) to be comparable to late Wimpy and Medi, the second (Dodgy) to Danger, the third (Gloomy) to PSYCHO, and the last two (Painful and Ninja), a new standard in difficulty as far as I'm concerned... that's going to be hard work, but a lot of people felt even most of PSYCHO was too easy, so... =P
I'm also trying something a bit different with the music rotation: It has 15 tracks, but rather than rotate the same 15 tracks throughout the whole game, there's instead a different rotation of 3 tracks for each difficulty ranking.
Finally, one change I plan on making - and yes, I am very aware this is going to be controversial, but ultimately it's my pack and I like it this way better - there will be no more timers on bombers. Bombers will explode instantly (they'll still have the "Oh no!"). However, since it exists solely for effect (since I don't make levels that rely on it), use of the nuke will still apply the 5 second countdown.
So far, I've only got 6 levels completed... four for Cheesy, one for Dodgy, and one for Painful.