Elemental representation was a worry that I had with the Prom/Rel (promotion/relegation) system, and is a valid concern. It's worth noting that with approximately 1 war per year, this would optimistically have 2 smaller wars a year, but each element would feature in at least 1 war. I will discuss some of the questions that I remember from chat below, but first I would like to discuss some of the problems I have with a dual element war.
It's pretty much impossible to prepare for. With such a drastic change to the format, the first war will almost certainly be disgustingly unbalanced. This is fine if it were only for 1 war, but even 2 or 3 wars down the line, I feel like some of the kinks will still be ironing out. The decision to choose the pairings needs to be more than "first place with last place", "opposite elements", or something similar, else you're going to end up with one or two pairings being horrendously stronger than others, while others would be much much weaker.
Assuming the pairings change each war (and I don't see why they wouldn't unless you go opposite elements), a crapton of work would need to go into balancing the market. And initial market prices are almost never perfectly balanced. Trying to balance for dual elements is worse than trying to balance for mono elements as well. It would be that much a hassle that you'd honestly need to get rid of Market (which is what some want anyway, but still). However it does go back to the thing of "how are elements decided"? As to how much a nightmare market pricings would be
Last thing I'll note, I feel like
" "Dual elements" or "different division" just dilute the original idea." is grossly underplaying. I mean it's bad enough that you'd need to share your win~ But the idea is more like a PvP Event than a War Change. Solo is almost always more popular than Duo, assuming the duo exists at all. Tennis being the primary example. Solo accomplishments feel better than duo accomplishments.
Minor note but the banner would be weird, but you could probably manage the banner being one element and the icon being a different element, or something similar.
Onto a few questions about Prom/Rel that were asked
Will the winning teams stay the same if they promote?
It would probably be optional. Players can opt in to stay on their original team, else they would be put up on auction.
WM workload?
I have never been a WM, so some of the stuff here is from Math and Asdw in chat
Most of the work would be done for the first war of the year, presumably. Rules would remain the same, but a few unbalanced things could be ironed out. Vault Tools are a pretty hefty problem, but the workload is reduced by a lot with the introduction of GDocs, and they could probably be optimised further. With less teams, the workload of individual wars would drop, and the war in total would only really have to go for 10 rounds or so, maybe less maybe a little more depending on rules and such. WM's could switch between wars, and you can still hire the pre-War Warmasters, as was mentioned elsewhere in this thread (which is an idea that I fully +1).