1. Trailers are all about misdirection. Coincidentally, Varys is also all about misdirection. He's the only "player" in the whole show who we're in the dark about; it's kind of a big deal. Unsullieds have been wondering for years what's the deal about Varys and what he really wants. Spelling it out in a trailer like that would be a poor move.
2. What Varys says to Tyrion in the trailer is totally in agreement with ADWD. In the book, Illyrio never mentions Aegon to Tyrion - here's what he said about who to put on the Iron Throne:
Are your Seven Kingdoms so different? There is no peace in Westeros, no justice, no faith
and soon enough, no food. When men are starving and sick of fear, they look for a savior. They may look, but if all they find is Stannis Not Stannis. Nor Myrcella. The yellow smile widened. Another. Stronger than Tommen, gentler than Stannis, with a better claim than the girl Myrcella. A savior come from across the sea to bind up the wounds of bleeding Westeros. Fine words. Tyrion was unimpressed. Words are wind. Who is this bloody savior? A dragon. The cheesemonger saw the look on his face at that, and laughed. A dragon with three heads.
Then in the following Tyrion POV, Illyrio talks about how great Dany is:
Next you will be offering me a suit of magic armor and a palace in Valyria. If Daenerys is no more than a sweet young girl, the Iron Throne will cut her into sweet young pieces. Fear not, my little friend. The blood of Aegon the Dragon flows in her veins.
Viserys was Mad Aeryss son, just so. Daenerys
Daenerys is quite different. He popped a roasted lark into his mouth and crunched it noisily, bones and all. The frightened child who sheltered in my manse died on the Dothraki sea, and was reborn in blood and fire. This dragon queen who wears her name is a true Targaryen. When I sent ships to bring her home, she turned toward Slavers Bay. In a short span of days she conquered Astapor, made Yunkai bend the knee, and sacked Meereen. Mantarys will be next, if she marches west along the old Valyrian roads. If she comes by sea, well
her fleet must take on food and water at Volantis.
In the trailer, Varys is simply doing THE EXACT SAME THING Illyrio did in ADWD: Talking about how Westeros need a savior and how Dany is the greatest thing since sliced bread. Neither of them mention Aegon.
Then there's also the aspect that Varys doesn't actually go to Meereen, and if Kevan is in, he most likely goes back to King's Landing before the end of the season. Meaning... what is causing this?
I doubt Young Griff is in, but until Varys talks about supporting Dany in 5.10 there's still plenty of room.