After seeing some posts about "Behemoth" (4+ CMC) and "Tiny Leaders" (3 or less cmc) as variant formats, I just had a quick thought and I am sure something like this may have already been mentioned.
What if we just used cards with CMC = 4 only?
There are over 100 legendary creatures that fall into this category, and according to gatherer there are 2800+ cards with cmc = 4. And the best part for all the kiddies out there? I am willing to let the Nephilim be legal as commanders (might need to find some kind of play on the number 4 for the commander title *shrug*)
I'd say it follows the B&R lists, but cutting it down to 2800 cards might pretty much take care of the cards that are on that list.
Any cards you think might be too powerful?
I think it can be something done pretty quick and easily, but still be fun and let you play with some hidden gems that either cost just one too much for Tiny Leaders or are sometimes outshined by some really good 6-drops.
The Kirin cycle (from Saviors of Kamigawa) might be too powerful.
Any four-mana spell with transmute is Diabolic Tutor for three, which seems a bit much. Fleshwrither is similarly powerful. Droning Bureaucrats can lock down everything if you have the mana. Hisoka, Minamo Sensei is basically unlimited 2U counterspells in this format.
These are the potential problem cards I can think of, off the top of my head, but this is a casual format I might be able to see myself playing.
Celestial Kirin destroys itself with every spell cast as well. Infernal Kirin does seem like a strong option, however.
I feel like abilities like Transmute are a feature of such a format. You have to figure out which cards you can play that do something before turn four. Droning Bureaucrats seems fine, it's expensive to activate and can't stop them from attacking AND blocking.
Hisoka by no means seems overpowered.
Transmute is the biggest problem because it is only a Dimir mechanic. If other colors had it, then they could a use it and things would be fine. I don't think either of the Kirins are too broken. Hisoka sounds great and a lot like a commander I could see myself building around
The Kirin cycle does seem interesting as there are also 84 cards with the type "spirit" (not counting changeling).
Not too worried about Transmute cards at the start as tutoring is just a part of the format, and yes it does give you something to do before turn 4. That will be most of the fun is trying to find the things that cost four, but still let you do something else for less or more.
Droning Bureaucrats is one way to think outside the box, if anything I would be more concerned with Humility.