Just compiling what I can remember here.
Adding Stunty Leeg fanmade extension to BB2 (playable in MP leagues, of course), as a DLC. Here is rulebook: https://fumbbl.com/files/Stunty_Leeg_Handbook_v1_0_1.pdf
Afaicr, it uses the same core mechanics as regular BB, with very little changes (a bit more Secret Weapons, they are assigned some "danger categories", and depending on those have different chances of ejection; some prices were changed a bit (bribes, apo), and more apo rolls are available throug inducements; I also think a skill or two was added, or may be just renamed a bit for fluff - but that's about all). It's trully amazing how they managed to create 16 new teams and a BB experience that actually feels different (even more madness and violence on the pitch) from a regular "big" BB - by just rearranging standard pieces from original rulebook! Those rules undergone a lot of testing for years since they were created, and should be pretty safe to add as a side-project. Perhaps, a standalone game on itself? Or just a separate option in the main client, withe their own leagues?
The main investment here of course would be a whole multitude of different models, as a lot of races and creatures in it are new, and I understand that's a huge amount of work. Still, that would be a great big 40$+ DLC (a set of DLCs?), if Cyanide'll deliver.
Squigbowl. Rules are here: www.enos.ltd.uk/Downloads/sqb rules.pdf (another one: https://www.thenaf.net/wp-content/uploads/2013/06/SquigBowl.pdf) It doesn't change that much at all (new "ball" models, new mechanics for ball running around and biting you :) ), but it could add a lot of fun and variety to league's life. Shouldn't be too hard to implement as a little nice DLC adding an optional tournament to Leagues (and to solo play campaign perhaps).
League auction. Below is a quote I've seen in some discussion:
This particular league has an auction mid season. We bid our gold for things like daggers, chainsaws, magic helmets, and various other oddities our league commissioner comes up with. I had a goblin with a dagger for a while until people decided he had to die.
Seems like a pretty neat idea to me, to be an optional feature for Custom Leagues. First of all, I don't really like idea of equipment in BB, mangles with the core mechanics too much. But 1-2 pieces of such per team won't be that big deal, I guess. And such autction will enrich gameplay significantly. Items can grant something as simple as stat boosts (or STAT1+ and STAT2-, at the same time), or something a bit convoluted, like turning one your player into a secret weapon player with addition of Stab or Bombardier skill. I believe card decks from CRP (not implemented in BB2 atm) could be a great source of inspiration for actual effects.
To heat it up a bit some struggle for those artifact could be implemented too, like in case of killing/BH/MNG a player wearing it on the pictch it could become property of the opposing team after the match (limit of 1-2 items per team still remains, so suprluss items must be transeferred to auction right away). Owner of the item also must select a player to wear it if he chooses to keep the item (so no hiding it in the treasury forever). Item should increase team's TV as well.
A "True Madness Unleashed" set of optional features for the new Solo campaing and Multiplayer Leagues. Sort of package of different Home Rules (previously proposed League Auction could be moved in this one too) released as another DLC. Much more randomness than in usual BB play. Like a lot more of weather-related events, with lightning striking the field randomly, mist covering it and limiting your view distance to 5-10 squares, strong wind gusts shifting players on the field in the midst of a game etc.
I also would like to see Stadium Enhancements that actually mean something, adding different bonusess, but also (like always in BB) having some downsides too. May be even some objects on the pitch you can interact with, for more spectacular gameplay (a pool with sharks in the center of it? some cannons you can use to hulr a ball or yourself to the other side of the pitch? A jump platform changing its position each drive? whatever)
Adding cards from LRB6 rulebook would also be great. Still I would change rules a bit somehow limiting player's ability to choose what deck to draw from, as unfortunately some metas are already devised cause some decks more useless than others, so only few of them are actually being used. That reduces variety of situations and is not very good for fun (and all those features are clearly is not for a serious tournament games :) )
Another nice bunch of Squig Bowl's (Ball?) home rules: http://www.talkfantasyfootball.org/viewtopic.php?f=16&t=36241
Cyanide/GW should really cosider adopting SquigBowl. In comparison with DungeonBowl released for BB1, it's really fun and popular by players. And needs much less work to bring it in.