Okay, here's my new potential list (pre-trimming-of-excessive-number-of-levels). Removed levels from the original list have been struck out rather than being completely cleared. New additions are bolded. Those with a > next to them are the ones I'd be most likely to drop when trimming the number of levels. Ultimate decisions will probably depend on not only the quality of levels, but also where the most gaps are in the difficulty curve (as you may have noticed, LPF has a very steep initial difficulty curve - going from Fun-style levels to Taxing- or maybe even Mayhem-style ones in the first rank).
>Glimmer 6 "A Package For Mr. Lemming"
>Arctic 1 "Generous Inclination"
Arctic 3 "23:58 12/14"
Arctic 5 "Dream Parade"
Breezy 20 "The Nuclear Plant"
>Puzzling 7 "More Than Just A Dream"
Puzzling 14 "So This Is How It Ends..."
Puzzling 30 "Drop Them Lems"
Perplexing 5 "When They Need It Most..."
Perplexing 7 "Dial "L" For Lemming"
Perplexing 12 "That's Just How It Is"
Perplexing 16 "Wiring Defect"
Perplexing 30 "Lemming Treety"
>Mental 1 "Be A Tidy Lemming"
>Mental 12 "Cave of the Cactus"
>Mental 13 "A Different Kind Of Hero"
Mental 16 "Skill Factory"
>Mental 21 "Adventure Time!"
Mental 24 "Branch Out A Little"
Smooth 14 "Cogs of War"
Smooth 20 "Over The Edge"
Bumpy 5 "Space Race"
>Bumpy 9 "Mechanical Split"
>Bumpy 12 "Luck of the Draw"
Bumpy 17 "Lem Your Lem Could Smell Like"
>Twisted 5 "The Cooling Vat"
Twisted 14 "A Universal Production"
Twisted 16 "Don't Hold Me Back!"
Insane 6 "The Mystery Machine"
Insane 10 "Comradical!"
Insane 14 "Artificially Sweetened"
Insane 19 "Overclocked Lemming"
This is only 29 levels total, even without removing any of the "potentially remove" (only the definitely removed ones from the original list). To try and keep a similar structure to the original Lemmings Plus Flashbacks, I'd like to go for 32 levels again. So, basically - which other three levels should be included?
Previous suggestions that weren't put onto this list are:
Puzzling 4 "Not Very Lemming Friendly"
Puzzling 13 "The Martian Lake Beast"
Twisted 17 "Climber's Edge"
Perplexing 21 "Wirebreaker"
Perplexing 22 "Pick-a-plan"
Perplexing 26 "Whisper of the Wind"
Mental 4 "Trade of the Tricks"
Mental 17 "Recycled Lemming Syndrome"
Smooth 13 "Trophy Cabinet"
Twisted 1 "Proceed With Caution"
Twisted 2 "Lands of Despair"
Of these, the first three were suggested by the second person; the rest were suggested by the first, and disagreed with by the second. However, Puzzling 4 presents an additional problem - its repeat is already in there.
So, adding Puzzling 13 - they make a good case, it has an especially unique value as an artistic level. Of the "controversial" suggestions, I do see myself agreeing with Mental 17. This brings us to the following list, which needs one more level (assuming no removals occur) to reach the desired 32 levels. You're still welcome to argue for the removal of a level, of course.
Holiday Lemmings Plus (4 levels)
Glimmer 6 "A Package For Mr. Lemming"
Arctic 1 "Generous Inclination"
Arctic 3 "23:58 12/14"
Arctic 5 "Dream Parade"
Lemmings Plus Omega (16 levels)
Puzzling 7 "More Than Just A Dream"
Puzzling 13 "The Martian Lake Beast"
Puzzling 14 "So This Is How It Ends..."
Puzzling 30 "Drop Them Lems"
Perplexing 5 "When They Need It Most..."
Perplexing 7 "Dial "L" For Lemming"
Perplexing 12 "That's Just How It Is"
Perplexing 16 "Wiring Defect"
Perplexing 30 "Lemming Treety"
Mental 1 "Be A Tidy Lemming"
Mental 12 "Cave of the Cactus"
Mental 13 "A Different Kind Of Hero"
Mental 16 "Skill Factory"
Mental 17 "Recycled Lemming Syndrome"
Mental 21 "Adventure Time!"
Mental 24 "Branch Out A Little"
Lemmings Plus IV (11 levels)
Smooth 14 "Cogs of War"
Bumpy 5 "Space Race"
Bumpy 9 "Mechanical Split"
Bumpy 12 "Luck of the Draw"
Bumpy 17 "Lem Your Lem Could Smell Like"
Twisted 5 "The Cooling Vat"
Twisted 14 "A Universal Production"
Twisted 16 "Don't Hold Me Back!"
Insane 6 "The Mystery Machine"
Insane 14 "Artificially Sweetened"
Insane 19 "Overclocked Lemming"