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Proposal: Random arena with increasing group size
Introduction
We all know and love the immediate gratification of the random arena, and its 4 vs. 4 goodness. However, many have pointed out that there is scant likeness between combat in the arenas (even in the team arena) and the 8 vs. 8 combat, as it is seen in the Hall of Heroes tournament and Guild battles. This suggestion is meant to somehow bridge that gap - the players participating willing, of course - by a modification to the random arena.
Implementation
A new arena for random battles needs to be made (or the old one scrapped, but that would liely be impopular), the rules from the old random arena kept by-and-large, however after five consecutive wins, a team will get another random team member. The now five man strong team will continue battling against other five man teams. After another five consecutives, the team will become a 6-man team, and so on until the team reaches the maximum of 8 players.
If a team of more than four players were to loose, they remain in the arena, but loose the latest added member (in fact, being "demoted" to a level lower of play). This is because the number of teams with many members are likely to be small, and would be unsustainable of the defeated team was kicked out entirely. If a team of four looses, the team it kicked out of the arena as per usual.
Discussion
As an added bonus, one might think that a successful 8-man team could be injected into the Hall of Heroes tournament after a set number of wins. This might be a first introduction to that part of the PvP environment, and will make sure that the team is working well. This might also be seen as a kind of interesting way to pick up a team for the tournament, instead of screaming "Exeprience pro trapper looking f grp. No NOOBS!!11!!one1!!"

The "kick latest player" function might seem harsh, but it is in fact a critical part of the implementation - It allows a team to get a "bad seed" and still survive inside the arena. Kind of an autokick of bad or mistmatched players, allowing the team another shot at glory.
A real question is if there are enough players in the random arenas to support this kind of playstyle? There might be, or there might not be. Only Arena Net can answer that one, but I suspect that, all regions combined, it might just work.