No way it will go to 5.8. I could see a 5.5, with the last patch starting the story for the next xpac, with maybe a random boss like the last one they threw in at the end of WOTLK that dropped OP trinkets just to keep players alittle busy.
I expect the PTR for 5.4 will take longer than the 5.2 PTR, because not only is there an entire new tier to test, there is also virtual realms and flex mode raiding. 5.2 was on the PTR for nearly 3 months, so I don't think a ~4 month PTR for 5.4 would be that unreasonable, pushing it to some time in October. I can't see the new expansion going live in less than 6 months after it is announced at Blizzcon. However, if you look at a time table like this.
Mid-October - 5.4
Early-February - 5.5 (lead in patch, possibly a RS type raid)
Late-April - 6.0 (new class mechanics, etc)
Mid-June - New expansion
A time table like that would be close to in line with their release schedule to date and shouldn't leave content too stale for too long.
"Pandarian Gladiator" doesn't have a ring to it, end of story. /thread
It's not strange at all that we have no new info on the new expansion, the Blizzcon before the next expansion is almost 5 months away.
---------- Post added 2013-06-17 at 09:34 PM ----------
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