An anti-roll head is another good idea.
There shouldn't be an issue making 21mm bodies designed to work with the older style Hound Dog / etc. heads?
Understood the point about the tailcap. Ultimately some kind of tailcap standard is necessary.
The old 18mm tube was created out of boring Surefire 6P and 9P bodies. Then the 18mm tubes became available from other makers. First FiveMega, Leef, etc. then Malkoff. You no longer had to bore Surefire lights. Then the issue of 123 compatibility became moot. My Hound Dog 18650 doesn't run well on 2x123, not on high. I have the 4x123 Hound Dogs as well and never ran primaries in them either, so backwards compatibility with 123 is a moot point.
Point is, 1x21mm or 2x21mm makes a lot of sense.
pay attention here:
Right now you already can buy 21mm lights from SOB (some other brand). If Malkoff does not come out with one soon, other makers will eat his lunch. You have to constantly innovate, stay with the market, or be left behind.
There are increasingly fewer reasons to buy 18mm lights.
Come to think of it, the only lights that make sense at this moment are 21mm bodied lights and also the ones that take only 123, for traveling. Like Surefire G2x Pro. (Yeah I know it can take 17670, but) When I travel, I don't care for recharging and dragging a charger or heavy rechargeable batteries with me, at times primary cells have their charm and convenience.