Seems like every time I look at the SL board there's some other established feature being completely overhauled in a way that would break existing content

Right, but - as you yourself stated - now's the time to make such changes, before too much content is created/ported over. Once I see that people are making content for SuperLemmix, preservation will absolutely become more of a thing even if it still won't necessarily be enough on its own to swing an important decision one way or the other.

you may be applying an expiration date to early SuperLemmix content because it's too difficult to predict how things will behave in SuperLemmix 3 months from now, let alone a year from now

In the time I've been active on the forums (since 2019), I can think of at least 3 physics changes that were made in NeoLemmix which broke existing content (Jumper & Shimmier exit behaviour which broke a couple of my own levels, Swimmer reaching bottom-of-water trigger behaviour which I requested, and a Glider fix which didn't affect me but I know affected others). There are probably more besides.

And before my time, there was "the great cull" which I know affected a great deal of content, and which even broke an official level from the OGs.

My point being: it's not unusual for even a well-established engine like NeoLemmix to make content-breaking changes, even years into its development and with a healthy back catalog. Far from putting anyone off making content for it, it continues to be the most popular lemmings content creation platform on the Forums - nay, the internet!

You can't make an omelette without breaking a few eggs.

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With the above said, here's a quote from this topic:

it's been said from the beginning that the SLX development isn't concerned with previous (Neo)Lemmix content; this isn't to be dismissive or inconsiderate of the work that has gone into the many fantastic NL packs over the years, it's so we can have something of a fresh start and generate new ideas free from having to worry about existing content preservation.

What do people think?

Please note - My own content is counted along with everyone else's in this equation; I'm not going to base any decisions on whether my own packs would be broken either.

The thinking here is not so much a push away from existing content than it is a pull towards new gameplay opportunities. It's absolutely nothing personal against anybody's content. Besides, anything that's worth converting will eventually be converted, either by the author themselves or by players who want to see their favourite existing packs in SuperLemmix; in fact, it's already happening!

If a majority of people on the Forums disagree that existing content should be mostly ignored, then I'm happy to reconsider my approach and perhaps give a bit more weight to that viewpoint when making any changes in SuperLemmix (particularly those regarding physics).

However, conceptual and/or aesthetic reasons for/against a particular behaviour will always be more substantial IMHO, since content can always be updated (and fairly easily in the majority of cases). Conversely, if a decision is made regarding physics that ends up being a questionable or outright bad one, then - if we take existing content as a big enough reason not to change stuff - we're all stuck with it.

Thoughts?

You should have an idea of which features are still subject to change and which ones are pretty much set. Currently, the impression that I've gotten is that EVERYTHING is on the table

Yes, that's correct; I'm open to ideas about most if not all aspects of the game, player and editor at present. The only exceptions are decisions that have already been made since the project began, but even these are still open to feedback (note that I re-implemented the Spawn Interval option - not sure if you knew about that?)

Having said that, it doesn't mean that all ideas will be accepted (although many have been so far), just that they will at least be considered.

If you wanted to make a SL level without worrying about if you're gonna need to make a ton of maintenance updates

You're being a tad hyperbolic here. Sure, the changes I've made so far require a fair bit of porting for existing NeoLemmix content. But any SuperLemmix content made so far would likely only require minor tweaks, if anything at all.

which subset of features are safe?
...
which skills/objects/mechanics/etc. are you considering making changes to?

At the moment, anything for which a discussion isn't currently open is relatively safe, and everything on the wish list is what's planned. But, ideas are coming in all the time so it's worth keeping an eye on this.

EDIT: Maybe I should make the wish list more prominent on the boards?

Consider this a period of experimentation where SuperLemmix is finding its feet. From your suggestion that I should be clearer about the goals, I will try to get together a more comprehensive "planner" which maybe specifies some general time periods, or perhaps rather what can be expected with each release version from here on out. Namida has been doing that for NeoLemmix for a while, and it definitely seems to help.

Keep in mind that there were very few breaking changes in NL history

Hmm, no. NeoLemmix made many breaking changes, let's say it like it is. It's not a bad thing; it did what it needed to do to keep the majority of the community happy and ensure its progress. SuperLemmix has the opportunity to do the same, but without the pressure of keeping the majority happy (I envision that NeoLemmix will always be the "main" engine, and SuperLemmix will replace Lemmini/SuperLemmini as the "worthwhile alternative").

There certainly were some, but they weren't something to be taken lightly. We did it on a few occasions when we decided the benefits outweighed the costs
...
So when you do break levels, you should have a good reason for doing so

To the first point: yes, I completely acknowledge that none of the changes made in NeoLemmix were taken lightly. In fact, its existence depended on being able to serve the majority of the community (as previously mentioned). I can imagine that no decision was made without a lot of back-and-forth discussion - a quick search of the Forum archives attests to this as well!

To the second point: "because it will improve the game" is a good enough reason for breaking any level, as NeoLemmix has proven countless times. SuperLemmix exists because some people felt that certain decisions didn't improve the game, and because (more recently) ideas are no longer being considered for NeoLemmix, so there is no chance of those decisions being reconsidered.

but we also weren't constantly discussing a new set of breaking changes every week or so (except arguably the infamous culling frenzy, but they were also pretty grouped ...)

Consider this an "updating frenzy," then, similar to that culling period in NeoLemmix's development history; I doubt that the ideas will come as thick and fast once things have settled down a bit. Really, the thing I've been waiting to type throughout this post is that: of course, there will absolutely come a point where less changes are being made, and it will become more about maintaining and bugfixing the features that have been implemented thus far. At that point, it will make more sense to create content for SuperLemmix. It probably isn't even that far away: the wish list only has one more physics-related change (the one regarding steel).

Generally, we don't want to be constantly revisiting old levels to update them for breaking changes. It takes away time that we can use for other things

Yes, fair point. This is duly noted. What I would say, then, to anyone reading this who is considering making content for SuperLemmix: perhaps don't make any full packs just yet. We have the Sandbox collection, which I would encourage you to get involved with instead in the meantime.