[SPOILERS] Revelations from Patch 3.3

[Cilia]

For a rival...

We don't need Lahabrea to fill that role.

That is what the Warriors of Darkness are for. The Quest Log explicitly states upon our first meeting that, given they clearly aren't friendly, they must be rivals to us. I'm expecting them to become recurring story bosses in 4.0. (Hopefully. Any game is made better with a good rival, and I really want to waste Darklander's smug ass.)

Alternatively we have Ilberd, whose ideals and motives clash with ours despite our similar goals. He was never decisively dealt with and remains at large; the fact 4.0 is implicitly taking us to Ala Mhigo just increases the probability of this eventuality.

For the Eyes...

It's nebulous exactly what is going to happen. Nidhogg is gone for good, that much is absolutely certain. Anything else remains open to speculation - that is, anything is possible.

It's possible everyone is just going to leave them where they are.
It's possible the Ascians will retrieve them for some end.
It's possible they'll become batteries for Omega.
It's possible we'll be named an Ishgardian duke with the Eyes hanging over our fireplace.
It's possible Raubahn will get a magitek prosthetic arm that uses one as a battery.
Etc.

Dealing in possibilities is unwise for reasons explained with the Ancient Aliens .gif. Anything is possible.

Possible < Plausible < Probable. Thus far I do not consider it probable that anyone will retrieve and use the Eyes... I acknowledge the possibility, but aside from an Ascian retrieving them it strains plausibility, and I consider it probable we threw them away so the story would not become "Who can control Nidhogg's Eyes?" since they have so many potential uses as aether batteries. If we just relied on dragon eyes to solve everything we'd have a hard time moving past Ishgard due to the city-state's history with dragons, and if the villains relied on them we'd have almost no chance of stopping them without our own... and clearly we're moving on, probably to Ala Mhigo, which aside from the fact their last king was a tyrant and they've been under Garlean occupation for 20 years, we have little to no information on. (Probably no dragons though.)

Addendum: If the Ascians were so desperate for dragon eyes, they'd just go get Tiamat's. There is literally nothing stopping them, and yet she's been left alone on Azys Lla for +/- 5,000 years.

For Blanhaerz...

... he was probably just observing. I think. True, the final scene was after the battle and implicitly after Ishgard passed their reforms, but he's just standing in Coerthas, looking at the Steps of Faith, and talking to Darklander via linkpearl. Elidibus tells Darklander all is proceeding as written, then Alisae is caught eavesdropping and saved by Thancred.

However, just before hanging up Darklander tells Blanhaerz he doesn't need to remain where he is and they can meet up in "the usual place," meaning either he already had the Eyes or an Ascian is going to retrieve them at a later date. The latter is more likely given the chasm is seemingly bottomless, but... again, possibilities.