All of this expansions arenas would reward you with a new title track (that would ultimately increase your faction cap) "Arena Title" (for lack of a better title track name).There would be Merchants that would sell you items (weapons, armors, etc...) that were sold for fixed amounts of faction (therefore if your faction cap was lower than a particular item you wanted, then you would need to raise your cap as well as saving up the faction to buy the item). This title track would get progressively harder to increase in ranks in the upper tiers, and the various arenas that award the Arena points would vary in point awards based on their level of difficulty.
The Character Slot: You would be granted one additional Character slot with this expansion that would be a PvP only character, but it would be a special character that could change Primary and Secondary class at any given time. This special character would have all the skills unlocked for play in the new PvP expansion only, play outside of this expansion would require that you own the corresponding chapter of Guild Wars and have that skill unlocked.
Arenas: There will be new arenas as well as test/practice equivalents of those arenas. In the testing equivalent you would be able to setup multiple team builds that would be randomly selected to oppose you and your teammates on various maps to simulate expected battles in the arena you are choosing to practice in. For example I could choose to play in an HA like arena which would have all the current maps/mechanics. I could then setup several meta builds that would become random encounters as I progressed through the various maps of HA. This would be great for beginners and pros alike as it would give you some indication of how certain skill bars play against the current meta builds (though limited by Hero AI on the opposing teams)
1) Tomb of the Primeval Kings Arena: This would be the old Heroes ascent and contain the exact maps that were used back in the day. The mechanics would be the same as they were as well (we all know we loved Altar Cap) This arena could POSSIBLY be limited to Prophecies only classes/skills.
2) 8v8 Random Arena. This would include new maps built for 8v8 random teams. Leavers would be punished by decreasing in your arena title track.
3) PUG "GvG" style arena. This would be all the current and future GvG maps but made available for Puging. This arena would offer fast advancement in your arena title track for wins, but at the risk of losing title track points for losses. Daily and Monthly leaderboards would be available to view top teams during the given timeframe based on wins/losses (these teams would be identified as a "team" by having 4 members that were always present in order to accumulate points for the leaderboard) Top 10 leaderboard teams would receive additional Arena points for that timeframe.
4) True 12v12 Organized Alliance Battles. Self explanitory really, would award you with alliance faction as well as advancement in your arena title track.
5) Sealed Deck Random and Team Arenas. Using current maps, you would enter the arena and talk to an NPC that will grant you a Primary class and a choice of 2 secondary classes with a random selection of skills (maybe 30 to choose from 20 primary skills, and 5 of each secondary) you could then advertise your skill bar in the TA version and get in a team with some synergy, or you could click the enter mission button on the RA version. EDIT: (Forgot to elaborate and thanks to FelixCarter for pointing this out) but you would be given a skill set that would be active for a given time frame (1/2 hour or whatever would be acceptable) which would work as follows: A) you go to the NPC and get a skillset. These skills would be "forced on you in the Sealed deck outpost for the next 1/2 hour. B) While in battle you will retain this skill for as long as you remain battling (consecutive wins can streak for a long time) C) When you return to the sealed deck outposts you can adjust your skill bar using your current skill set until that 1/2 hour has expired. In which case you must visit the NPC to gain a new skill set. Something like that.
6) Draft style tournaments 4v4 where each player is given a set of skills to select 1 skill from, then pass the remainder of the skills to the player next in line until all the skills have been chosen.
None of the arena's in the PvP expansion would offer observer mode unless granted permission from both participating parties. (high ranked teams would be presented with an option before the battle started if it was OK to display the events about to take place.
And finally a little bit of PVE flavor splashed in. There would be a zone that has all the statues of the gods. Youre special PvP character would be able to form parties in this zone and would be able to travel to the Fow/UW if their region had favor, which would be determined by winning battles in the Hall of Heroes within Tombs of the Primeval Kings

I didnt have much time to write this up so sorry if its lacking details, I am hoping that suggestions are made and this can be fine tuned, but really I doubt any of this will ever happen, its more of wishful thinking really.