Train in various types of regular clothing.
Done that with my BJJ. It's still just BJJ.
Train in dim light conditions.
Done that. It's still just BJJ.
Train on stairs, in an elevator, in a vehicle, between vehicles, on the sidewalk, in a parking lot, in an alley etc.
Done that. It's a fun exercise and helps with mental flexibility. (Can't really spar full out on stairs without someone getting hurt though.) It's still 90-95% the same fundamentals with 5-10% situational adjustment. The important thing is to develop usable foundational skills and then you can periodically try them out in various environments to get used to adapting as necessary.
Don't train in a controlled, artificial environment, on soft and dry mats, with a single opponent that has agreed to abide by a rule set.
Every training environment has controls and rules that your training partners abide by. That's how we don't all end up in the hospital after every training session.
I think a better way of expressing your point would be "Don't always train in the same environment and with the same rule set. Mix it up and see what changes."
BTW - the advantage of working on mats is that you can train much longer and harder and thereby develop more functional skill. Suppose you want to be able to throw someone onto the ground. You're going to start with drilling the throw on a cooperative partner - hundreds of reps, thousands if you want to get really good at it. Then you're going to have go live, sparring against a resisting opponent. Lots of hours doing that and often the throws won't end up being as safe and controlled because your opponent is fighting you the whole way.
I think you're going to have a difficult time finding a training partner who is going to be willing to take thousands of hard falls on concrete.
Train with conventional and improvised weapons.
Done that. I'll freely admit that BJJ is an unarmed fighting art, so I draw from other sources (mostly FMA) when using weapons. I do make good use of my BJJ when clinching or tied up on the ground with someone who has a weapon.
Train against multiple opponents.
Done that. I do have to say that, regardless of the system you are training, fighting one against many is very low percentage unless you have superior weaponry or vastly superior skills and physical attributes. Usually the best option is to survive, disengage, and run.
Train to use de-escalation techniques.
De-escalation is really important and I've been moderately successful at it over the years. However, I've trained with hundreds of instructors from dozens of arts over the years and I don't think any of them had any special qualifications to teach de-escalation skills. I think it's hard to find individuals who have legitimate skills and experience in de-escalation and also know how to teach those skills in a systematic way. Finding someone who can teach those skills in a manner appropriate for a civilian self-defense context (as opposed to application for a LEO, doorman, social worker, etc) is even harder. Anyway, those skills don't necessarily have to be bundled with a specific martial art.
Train for situational awareness.
I've worked on that, although it really isn't something specific to any particular martial art. You can practice situational awareness with your Karate, Judo, Wing Chun, BJJ, Bujinkan Taijutsu, Kali, Tae Kwon Do, Boxing, MMA, whatever.
