Hi!
Fast forward and replay features are huge for reducing frustration. EricLang's level editing and playtesting program, Lemmix, also inclided frameskip and savestates, which might be worth considering.
In addition to adjustable release rate, these are good ideas. I can't see save states being a part of the main product, though.
In my opinion, a good game is always a good tool as well. Providing savestates would increase the user efficiency of trying new ideas in a level. I'd opt for anything that helps! (in singleplayer at least, heh. :-D)
I should probably also have mentioned that I would like to make a multiplayer option in some way.
If you're planning competetive multiplayer, I think I've got some interesting ideas to share. While writing
L++, which has networked multiplayer, there were lots of unexpected design corrections.
Although I use identical game mechanics for singleplayer and multiplayer, a multiplayer level calls for a very different skillset. The Amiga/SNES two-player mode used the regular skillset of Climber, Floater, ..., Digger. This is ideal for the singleplayer mode, however:
Lemmings control should be a little more direct in multiplayer than in singleplayer.
With more direct control, not every single level has to be a huge bunch of terrain, and we get a better chance to recover from other players' sabotage attempts. Thus, I implemented some skills from L2. As you are keen to improve upon what's already there in L1, such a concept might fit to your game as well!
Exploder with knockback - This is the main way to dislodge enemy blockers, at the expense of an own lemming. One can even use it to punish players who hoard all their lemmings directly under the entrance hatch, which could be very boring. Sneak a few lemmings in, BOOM, everyone's flying all over the place. :-D
Before I included this Exploder, we had to limit the blocker skill strictly. Blockers placed on steel were absolutely permanent, and if the goals weren't set on steel, you could simply dig away all the gound under enemy goals. Remember, removing terrain is easier and faster than re-building it. For this reason, the original Amiga/SNES levels often didn't feature blockers at all, which limits the level terrain possibilities harshly. For almost every terrain, blockers are needed to control lemming flow. But the stronger they are, the better of a strategy hoarding/trailblazing (using a single hero) becomes. Thus, this exploder regulates it again, which is what we want.
Walker skill - This works exactly as in L3: It reverts workers back to walkers, including blockers, or turns walkers around. This is the more direct lemming control mentioned above. It's pretty important if you need fast and accurate control of a few certain lemmings, which is common in multiplayer.
Jumper skill - This allows to extract a single lemming from a well-hoarded crowd, and generally makes it easier to get around. However, if we were going to include the Jumper in every multiplayer map, along with walker, blocker, knockback exploder, a lot of the eight possible skill panels are already full. :-) The map creator must be able to choose what skills to give, like for singleplayer levels.
More building skills - This should remedy the fact that it's easier to destroy terrain than to build up to new places. Platformer and especially Roper seem like ideal additions to a multiplayer game.
Level design - Every player wants to make a path from his hatch(es) to his goal. Thus, for maximum fun, have the players' paths cross each other - early, and often. Of course, give enough space and freedom to allow different strategies, but always make them interfere with each other by having crossed paths!
Revolution's connecting edges made things a lot more interesting, though I'm not sure there's any good way to present that with a flat display...
For multiplayer, this is very important because it balances the starting positions.
I don't know how to implement it the best way. I myself just scroll freely over the edges and let the stuff from the other side scroll into view, so that one can't notice any seam. Of course, this feels odd at first if the level size is one screen, but you can scroll freely in all four directions. :-D
-- Simon