I've thought about this idea from time to time, and it's probably not a bad idea. Perhaps I'll make something like that before working on LPIV Bonus Pack or my other planned project (won't say anything about it at this stage, apart from that it isn't a Lemmings Plus pack of any kind).
I don't think autosteel needs to be included - that's a feature that matters more to level designers than players, and thus would be more suited to go in the pack creation tutorial (which I should probably get onto soon, now that I've done the improved Flexi Toolkit that I had planned for ages but only recently got around to actually making) - but certianly the different steel mechanics (pixel-perfect areas, completely indestructible) should be illustrated.
What else should be included? My thoughts:
Skills - One level for each skill. The existing tutorial pack levels could probably do the job, perhaps in combination with LP Omega-style levels preceeding them (ie: one level that uses the new skill and nothing else, followed by one fairly easy level that uses the new skill in combination with some of the official skills).
Objects - One level for each type of new object (or at least ones that (a) do something, and (b) have actually been used in NeoLemmix content, rather than just existing for Cheapo compatibility). Co-dependant objects (eg. locked exits + unlock buttons, or teleporters + receivers) or related objects (eg: splat pads and antisplat pads) could possibly share a level.
Gimmicks - Not sure how nessecary this one is, but perhaps tutorial levels for (at least some of) the gimmicks, in particular those that actually change the gameplay rather than simply messing with the user's interface (so probably would not include one for, eg, Reverse Skill Counts, Overflow, Bait-and-Switch, etc; but would consider it especially important to include them for often-used and/or versatile gimmicks with a significant gameplay effect, such as Karoshi or Zombies).
Other - This is where things such as the changed steel mechanics could be illustrated. I'm not sure exactly what things should be in here, though.
I also don't know for sure if that's the best order. I definitely think the new skills should be illustrated first before anything else, but perhaps the "Other" should come before the new objects and the gimmicks, since my guess is it would primarily relate to things that did exist in classic L1 but work somewhat differently in NeoLemmix.