Speculation about the importance of Varys and Illyrio’s meeting in the Red Keep in AGOT in terms of the Blackfyre theory often misses that House Blackfyre wasn’t even mentioned or conceived of until The Sworn Sword. It seems mistaken to analyze their meeting and draw conclusions about the Aegon plot when that particular Aegon clearly did not exist in the story at that time (though as of ACOK the groundwork existed for a survival reveal, with the ‘mummer’s dragon’).

I hate to well actually, but the Blackfyres weren’t mentioned until A Storm of Swords, which came out three years before “Sworn Sword.” 

And Aegon is clearly set up in Dany IV of ACOK, both with the mummer’s dragon and her vision of Rhaegar and the “three heads.” 

Moreover, given that we know GRRM wrote up huge outlines of the Targ kings before he started writing the main series, I think the Blackfyres were there earlier than we think. Perhaps not filled out completely, but there was definitely a seed of an idea there.