We do have plans to add more battlegrounds in the future. How many, I couldn't answer for you at this time.It is interesting how the last paragraph is worded. The tidbit about the expansion is mentioned after the part about possible general PvP changes. Could this mean that the next expansion will focus more heavily on the Alliance vs. Horde battle? It may just be wild speculation, but I don't think Blizzard wants to get tagged as "lazy" by just producing another expansion featuring just ten levels of content.
There is a method to the madness, however. Mostly, it has to do with making sure there is a healthy population of people to take part in the battlegrounds. If we have too many battlegrounds, the population will thin out and queues will go up. Where Arenas are quick battles that don't need a lot of time commitment (based on limiting how many games a team may choose to play beyond the minimum), battlegrounds tend to take a bit more time and have no real downside for playing them almost exclusively if people choose to.
We have more plans in the future in general for PvP though nothing more concrete that I can share with you right now. It's common knowledge that an expansion is in the works. More on the details will come in time as to what all will be included in it.
However, Blizzard usually sticks to what they do best and rarely stray from the beaten path. On top of this, it does not make much sense to release a PvP expansion when so many players enjoy WoW's casual quest based leveling.
Hopefully Blizzard will be a little quicker releasing the next expansion. Even though I haven't touched The Burning Crusade yet, I still want to see what Blizzard has in store for everyone.