Hi there!
This is a neat sun team you have here, Ninetales supporting Heatran and Venusaur while you have Rotom and Landorus-T to help check Rain and Sand Teams respectively. One gaping weakness I do see to this team is Mamoswine, Mamoswine is very anti-metagame right now, having a certain niche to revenge Dragons and faster Pokemon weak to priority, as well as being a great offensive user of Stealth Rock, both of these reasons make Mamoswine a big threat to your team. Mamoswine can OHKO 2/3 of your team, while your supposed Mamoswine counter in Rotom might not be able to deal with it when locked into Volt Switch or Hidden Power [Ice]. For these reasons, I think you should change your Rotom into a
Defensive Variant rather than your Choice Specs set. Defensive Rotom does a number of things for your team, it gives you a solid answer to Mamoswine, as Rotom resists it's dual STAB and can OHKO back with Hydro Pump or cripple it with Will-O-Wisp -- either of these stop it dead. This puts a lot less pressure on the rest of your team to deal with it. A more defensive Rotom-W will give you a better check to Rain Teams that are very common in the new metagame, as you can burn threats that make it easier to switch in Ninetales or Venusaur which can make a big difference considering you aren't running a spinner. A more defensive set also gives you a lot more longevity, being able to Pain Split up against defensive threats to gain more health. The only thing you lose while running a defensive set over Choice Specs is the ability to shut down walls, which I will get onto in a minute.
As a result of running a more defensive Rotom-W set to deal with Mamoswine and Rain more reliably, you might have a little more trouble against defensive cores, which are even more common in 5th Gen with the release of things such as Regenerator Amoonguss . To deal with these defensive cores that might cause you some trouble, I think you could run a
Deoxys-D over your current Espeon. Deoxys-D does a number of things for your team, most importantly, it provides you with 100% reliable hazards which is something that Espeon cannot provide. While Espeon can avoid status and stops opponents setting up hazards, you will most likely not lead with Espeon. On the other hand, Deoxys-D is a reliable lead, and can also prevent hazards by using Taunt, making it a great stallbreaker. Deoxys-D also has a lot more bulk than Espeon while still retaining a great speed stat, not to mention it also has reliable recovery and still gives you the same synergy as previously.
Also, I second Expert Physics's suggestion of
Hidden Power [Ice] > Hidden Power [Fire] on Venusaur, for all the reasons he listed.