A little advice

[Mdpoyser]

Im working on an unseen seer build and here is what I would like to do

changeling
rogue 1/beguiler 4/unseen seer 10/ x 5

Im having trouble deciding what to do for x, my original thought was to do invisible blade for the boost to SA and BAB,

but swordsage might work as well with shadow blade and maneuvers that blend well with the beguiler spell list, and skill points,

then there's swashbuckler with daring outlaw, give int to damage which will be high for spells plus the better BAB

I think my new group focuses more on the roleplaying than the combat so im not worried about optimizing but I don't want to gimp myself at high levels

and until I hear any different from the DM all books are ok (hes pretty easy going)

[Waker]

Well, there are a number of options available to you.
-Did you take the Changeling Rogue acf from Races of Eberron? Makes being a social character so much easier. Not to mention that you get Trapfinding back from being a Beguiler anyways.
-Arcane Trickster (DMG) is a decentish choice after Unseen Seer as it progresses your spellcasting, sneak attack and skills a bit more.
-Prestige Bard (UA) would add to your spell list a bit and fit quite fine. You would lose access to a few spell levels though.
-Swordsage is a rather fine choice. If taken at the earlier levels, ask if your DM will allow Carmendine Monk or Kung-Fu Genius to apply to the swordsage. Both feats allow your armor bonus to be based off of intelligence rather than wisdom.
-Divine Oracle (CDiv) has some rather fun bonuses to add. Granting the Oracle domain is rather handy as well. Don't be fooled by the name, it can be entered by arcane casters.
-If you take the Racial Emulation feat from Races of Eberron, you could qualify for race specific classes, like the Shadowcraft Mage from Races of Stone. This is an extremely powerful illusion based class that works just swell with a Beguiler.