I don't think the Tyrells have a prayer of uniting with Aegon. Their fortunes are FAR to tied to that of the House Lannister for that. Margaery Tyrell was married to both Lannister Kings. Plus I think Varys want's BOTH the House Lannister and House Tyrell taking the fall for a broken realm so that Aegon can be the hero that puts it all back together again. Not to mention that Dorne (a very likely Aegon ally) Hates the Tyrells only a little less than they hate the Lannisters. I'm convinced one of the Sand Snakes are going to ultimately murder Tommen and Margaery. Though they've made it clear they'd love to take out Jaimie and Cersei as well.