I think the question still remains, if Varys is a blackfyre and he really wants aegon, then why is his plan so shitty? 1
However, if Varys is working for the Others using the facelessmen to get rid of dragons would still be congruent. But then, why give Dany the eggs in the first place? Since having Jaquen in place implies a fore knowledge of dragons, given Jaquen's Citadel scavenger hunt.
Honestly I did just come up with this "theory" (and was kind of joking, at first) but its much better than Varys being a secret blackfyre. But I can't believe he Varys ever believed he'd get the Dothraki to Westeros or that he could take over with 10,000 men. 2 Varys having been working for the Others all this time... would be so so so much better, think about it. His castration story, his claims to be causing chaos, his dislike of magic, since that's what's needed to stop the Others. 3 His nonsensical plans with various scions hidden away, Gendry, Tyrion, Dany, Aegon. It would finally make sense. All of it. And we know the Others can be bargained with from Craster. If Craster was 60 Varys could even be one of his sons, raised by the Others. Suddenly the Others haven't just be the peripheral force, separate from the Game of THrones, they've been in the think of it, for years. 4
1. How exactly is Varys' plan shitty? He's causing unrest in Westeros to discredit the ruling dynasty so people will accept the alternative he has stashed away (fAegon) more easily. The Dothraki are only part of the plan in order to cause unrest, they are not supposed to take over. Robert would have two choices: either hiding in his castles, discrediting him as a weak and incompetent king. Or facing the Dothraki on the battlefield, which would be quite risky, because it plays into the Dothraki's strengths. As for Aegon's safety: Yes, there are pirates on that shore. But usually, pirates don't kill people, they rob them. If you give them your goods, you're fine. And Illyrio would always be able to buy fAegon from slavery. Or to turn the question around: Given the assumption that Varys is a Blackfyre loyalist, what else would you propose?
2. As I said in 1., the Dothraki are not central to Varys' plan. They are one possible way of causing unrest in Westeros to discredit the current rulers so people will more easily swallow fAegon as their saviour.
3. Is really magic needed to stop the Others? Where did you get that idea? There hasn't been much magic for 150 years, a perfect time frame for the Others to attack, don't you agree? Instead they waited until magic got stronger to attack? That doesn't make sense - unless the Others themselves profit from heightened magic levels. Which would mean that no magic would be better for fighting them. And indeed, I don't see how magic has ever been truly beneficial in this series.
4. The general problem with your crackpot theory (no offense meant, I've had my share of those over the years, too) is that there's no established connection between Varys and the Others whatsoever, and your few weak links are even built on massive speculation (like magic being needed to defeat the Others).