I live in Dulwich so Denmark Hill and Forest Hill (for LBG) are both useful stations for me which is why I'm so interested in Thameslink and the rebuild.
My current reading of the Thameslink service pattern after 2018 is that up to 16tph will approach Blackfriars from Elephant & Castle, but only 8tph of this will go on up through the Thameslink core (joined by 16tph via LBG) in the peak.
Of the Elephant trains that proceed in to the core, 4tph will be Wimbledons (clockwise & anticlockwise), 2tph will be fasts from Maidstone East, and 2tph will be Sevenoaks services that will stop at DMK, Peckham Rye, and so on. The other 8tph will be a mix of Catford Loop, Rochester, Kent House etc.
Given DMK currently gets 4tph total in the peaks, I'd hope for a modest increase... to perhaps 6tph total (don't have any real figures for this, if it's even been decided yet), but as I say this would leave 2/3rds terminating at Blackfriars, but I suppose not very long to wait on average (not long enough for a Costa at any rate) if I can just cross to P2 for 24tph of Thameslink!