I saw a lot of people complaining quite vociferiously about this on the locked thread, and I can't understand why. From a lore PoV, well, nobody complained about lycanthropy, curing it and then being able to just speak to Aela to re-contract it come Dawnguard, did they....?
Anyway, I honestly think this could pan out in one of four ways.....
1. NEW skill trees with new perks. This in and of itself means the level cap will be raised so no worries, more perks to gain.
2. EXISTING skill trees with new perks. If this is the case, honestly, I think it's a bit unfair to those players who've sunk considerable time and effort into their character and are at the level cap (I fall into this category!). Fine, make me do a tough quest to reset my perks, make it a one-time-only deal, but please, don't freeze me out of new content!
3. INDEPENDENT skills trees a la Vampire Lord / Werewolf.
4. Something different, perhaps acquiring new skills by reading a specific skill book in that library, as opposed to levelling, acquiring a perk point then choosing to spend it.
Right now I personally feel option 4 is the most likely, in that acquiring these 'books of forbidden knowledge' will grant a perk point to spend as we wish when read.
Oh and hi; this is my second post here - long time lurker, long time player, never really got involved in the wider community until now. I'm really looking forward to this expansion - it does sound very much like it'll be substantially meatier than Dawnguard was.