This is a petition for Jagex to implement GIM/HCGIM options for the upcoming Trailblazers IV: Reloaded League event this holiday. Leagues are all about having fun your way, and I see this option for groups of friends as nothing but a benefit.
Players are already not going to be able to unlock the full map, and this would give groups of players interesting decisions on how to split regions of content/ or choose to group for other content (Raids, etc.) This would also allow the player base to attempt content they may otherwise never do so themselves, like a HCGIM group, something that is premier content on youtube/twitch but most players don't have the time/skill or want to invest in such a challenge.
As far as I can theorycraft, this will have 0 effect on players who choose not to GIM, other than HighScores, but these could be seperated similar to the main game. Also you should probably only allow presige groups as it is a limited time gamemode, and not worry about the GIM re-grouping features. Unless you feel this feature would be unfair to solo players in which a GIM grouping feature could be implemented with a diminished leave/join/trade timer, but this could run the risk of players group hopping for items, but this timer could be increased for every hop/or a scaling timer for players in higher tiers as a counter. At the end of the day it is still on players to vet new players to their group, and it's a League so the items have diminished value if stolen.
Jagex please implement this into the upcoming League, you are already recycling a concept, let us have a fresh take on it this way as well.