There are speculations that many major lords of the Reach have switched sides to Aegon like Lords Rowan, Tarly, and Golden company's "Friends" in the Reach. Mace will either switch sides to Aegon or suffer a defeat from JonCon because JonCon would not give him battle if he didn't think he'd win. Plus when Mace realizes the real threat of Euron and learn what he is doing to his own lands it's quite possible Mace would switch sides. Either way Reach will nbe for Aegon soon!
And as to the threat of Dragons, Dany's Dragons are not that big as Aegon the Conqueror's were and can be killed. And Unsullied are majorly a defensive force and they have never been tested against Armoured Knights. Sellswords and freedmen can be disregarded.
And finally the Dothraki! Though its not confirmed in the books but when the Lords of westeros hear that Dany is coming to unleash dothraki savages on them, the whole realm will unite to throw them back to the sea!
It's always hard to successfully invade against a foreign land without no help from within whatsoever! So when Dorne, The Reach, The Stormlands and the Crownlands will unite under Aegon's rule from the Iron Throne I think it could go either way! Maybe even Riverlands will join in because they have no love for the Lannisters and welcome Aegon as their King.'
And as to the threat of Littlefinger, he doesnt really have that much power. He is the Liege lord of the River lords but just in name, with the demise of lannisters down goes Petyr's claim to Harrenhal! And he is the Lord Protector of the Vale but will the Lords answer his call if he calls the Banners? No. Plus Vale would probably be involved in the North!
So to sum it all up, Long may he Reign King Aegon Targaryen, Sixth of his name, The Only Dragon that We Need!!