You can't really make a balanced game suitable for eSports-level competition unless you design everything around a very specific number of players per side. In Guild Wars 1, it was eight players and two teams; every step removed from that ideal was a huge degradation in the overall balance and level of play - Guild versus Guild was far and away the most competitive mode, Tombs was the next best, and the Arenas and Alliance Battles were pretty much passed over unless you couldn't get a group together for anything better, assuming that you were after actual 'competitive' play, and not just any PvP at all. (Fort Aspenwood and Jade Quarry were similar, for pretty obvious reasons, although I have to admit that I find FA to be good, dumb fun, the way I expect World vs World to be.)
The thing is, despite the fact that 8v8 worked exquisitely well in the ideal case, where both sides had full groups of players who were skilled both at working with each other as a team, and at individually being able to play their characters well, the ideal just didn't happen often enough to catch on the way they intended it to. Eight players who could play at that level was just too many for most people to be able to scrape together, practice, and play regularly as a group (which lead to all the people who could only really do well by grabbing a build from PvX or whatever and making everybody play their assigned role). It's a lot easier to coordinate yourself with four other people than it is with seven other people, and I think it's pretty clear that the main reason that the game is being balanced with an ideal of 5v5 is because they're hoping that top-level PvP is going to be accessible in a way that Guild Wars 1 really wasn't. The double-edged sword is, you can't really balance a game to a 'competitive' level for 5v5 without making it unsuitable for 'competitive' play at higher player counts.
I don't think you'll really find ArenaNet targeting modes that are in between the 5v5 structured tier, and the #XXv#YY unorganized World vs World stuff. If you want the game to be on the level of an eSport, you'd have to play 5v5 anyway, and if you're not looking for that finely honed of an experience, they're better off having you jump into WvW to keep it populated and lively.