I don't know if you're going to be seeing "better" items, per se. Not inherently better, anyway. Perhaps situationally better, or giving benefits to different situations which you would like to leverage.
I don't think you're ever going to have to worry about your L20 equipment being obsolete.
I'd be hopeful for new missions/mobs/quests, maybe professions/skills or a joint Guild Arena concept (i.e. You've met some friends through grouping together who are in their own guild U.S./Europe and you can form a superguild for PvP/GvG purposes.
Expansions will open further chapters which are vital story evolutions of guild wars, more cut sences, more missions, more areas, more enemy armies, more rewards. Dont worry about it, your armor and shit will still be as good as it was, just not new
It will be a while before an expansion comes out, so your top notch gear will be well used by then. And im sure you will have tons of money by the next expansion to upgrade
GW is a revolutionary game. I think we can agree on that. Not totally new, but certainly different. So to think that they're going to give us the same type of expansions that other games have dumped on us is naive. I think we'll have the same reactions to the expansions that we have to the game... good job!
I predict that the first expansion might be focused on PvE like people above me, simply because that's the easiest thing to do without upsetting playbalance. But future chapters will revolutionize PvP. They'll give guilds new GvG chalenges, and perhaps even let alliances of guilds form. I'd love to see an alliance vs alliance 100 vs 100 tourney. We'll see if they go that way.
But I hope we're not going to see the creeping doom that killed DAoC and EQ.
Gonna have to agree with Shagsbeard on this one, they are probably going to add to the challanges. With the level cap at 20 and no current plans to raise it arenanet is going to need to use a diffrent approach to keep people enthused with the game, and im pretty sure they wouldnt have it any other way considering all the MMO molds they broke when they created this game.