As much as I can be excited for anything in 5e, I am excited for the Warlock invocation changes. I had long liked the thought of a Celestial Tomelock, but lamented the loss of Hexblade that being Celestial would require. With 5.5e, I would be able to play a Celestial Bladelock at level 1 and pick up Tome shortly thereafter, so there would be no need to choose. As long as the Tomelock can still learn further new rituals (the invocation text is somewhat vague on that front) it would make for a supremely versatile, effective support character that doesn't hit the insane power levels of other full casters. The character would be useful in essentially any group (nobody minds some extra healing and solid damage), and being nearly 100% SAD means fears would actually be on the table rather than a distant, unlikely dream, particularly with Lessons of the First Ones in play.
Healing, ranged damage, 2x Extra Attack at 11, rituals, temp HP. The baseline of invocations is 4 (two pacts, Agonizing Blast, Thirsting Blade), with at most another two "needed" for Blade stuff (Eldritch Smite and Lifedrinker, and honestly I dunno if you need either), so there's room for other fun goodies, like Pact of the Chain (I just love pseudodragons too much), Gift of the Protectors, Ascendant Step, the aforementioned Lessons of the First Ones, all sorts of stuff.
And beyond that, I really do love the story potential of a Celestial patron. It's a refreshing twist on the very old Faustian bargain concept, and implies interesting things about both the character and the patron.