You are Viserys Targaryen, what do you do to retake the seven kingdoms for your house?

[Aline de Gavrillac]

Wait for Princess Daenerys to hatch her dragon eggs and then make himself useful.  Marry Arrianne Martell.  Befriend the Sealord. 

[Staenerys Stargaryen]

His best bet would have been to exercise more patience and ride a dragon with his sister but he was a total crazy and begged to be killed from the first time we met him

[Bullrout]

He should have waited.  Little Dany would have hatched those eggs eventually.  

[BlackLightning]

Treat Daenerys well; treat her as a sister, if not an outright co-ruler. I might spare her of a marriage to Khal Drogo. The Dothraki would be extremely useful but what would've been more useful is a marriage alliance. Dany would be a great match for Willas Tyrell, Oberyn or Quentyn Martell, Robert Arryn or, if I'm feeling daring and have some assurances, Victarion Greyjoy, Edmure Tully and Robb Stark. Maybe even Bran Stark, so as to make him both a prince and a hostage without interfering with the governance of the North by taking the direct heir to Winterfell.

I would also make it so that Daenerys would not only keep their maiden name in marriage but would be able to pass the Targaryen name onto her children. To that end, I would give Daenerys and her children sole rulership of Dragonstone.

Exercise more patience. Coordinate my own marriage with Margaery Tyrell.

[Lord Varys]

The marriage alliance nonsense isn't going to work. Viserys III would need an invading army and some victories over Robert's friends and allies to show the Realm that he meant business. Nobody would side with him over the prospect of a royal marriage if they didn't believe he could win - at least nobody on the level of House Tyrell.

Once had had established himself he could seal a crucial alliance with a marriage, but this wouldn't work before he had established himself. And he himself would be stuck with Arianne Martell, anyway. Doran would only support him if the contract was honored.

In that sense - the plan to work with the Dothraki and eventually the Golden Company and Dorne (assuming Viserys III had any inclination that they would eventually join him) wasn't bad.

Once he was in Westeros things would have depended on how good his generals among the Dothraki, the Golden Company, and the Dornishmen were. He himself was no general, he would have just been the royal figurehead, so the crucial thing would have been that he had good men to win his battles for him.

If they had moved at the right time Viserys III could have had tremendous success during the War of the Five Kings - Renly's people could have switched to him after he was murdered, the same goes for the Riverlands and the North after the Red Wedding (even before the chance to acknowledge a Targaryen king could have helped Robb to save face and give up his crown honorably) - since he would have come with fresh troops those people could have needed to defeat their enemies - the Lannisters.