Hey Dragonuser, very solid sun team you have here. Congratulations on all of the success you've had with it. However, I see a few things that can make it even better.
First off, I agree with everything Razza mentioned in his rate. Any half-way decent team with Chansey/Blissey completely walls you. Setting up Toxic Spikes is an unreliable way of breaking Pink Blobs, since all teams with Chansey are semi-stall/balanced teams and most likely have a spinner/poison-type to deal with the poisonous hazards.
With all that said, I second Razza's Refresh > Substitute on Latias
recommendation. However, it does open Latias up to Pursuits and keeps it from breaking Volt-Turn chains. So if you don't want to go that route, another option is Taunt > Hidden Power Ice on Heatran
. Taunt keeps Blissey from healing and allows Heatran to slowly wear it down with Sun boosted Magma Storms (you can also add a few more EVs into HP to tank Seismic Tosses.) Hidden Power Ice is expendable anyway, since it has very little effectiveness on Heatrans that aren't holding an Air Balloon. Without the pseudo ground immunity that Air Balloon provides, Heatran can no longer tank a hit from DD Dragons and is OHKO'd by Earthquake before it can even use HP Ice. (If you decide to use Refresh on Latias, then keep HP Ice on Heatran and give it an Air Balloon > Leftovers.)
Speaking of DD Dragons, Dragonite/Salamence also seem to be a bit of a problem; they get easy switch-ins on Dugtrio and can sweep after just 1 Dragon Dance. To remedy this, I'm going to recommend Mixed Growth Venusaur with HP Ice in the 3rd moveslot > Special Growth Venusaur
(you can play around with the EVs.) Yeah, the loss of the Special set's pure power hurts, but the ability to revenge kill Dragons makes the trade-off well worth it. (If you give Latias Refresh and Heatran an Air Balloon, I would leave Venusaur as it is, with maybe a few more EVs in SpAtk.)
HP Ice Venusaur also helps against Gliscor and Landorus, allowing Forretress to use Gyro Ball > Hidden Power Ice
so it can hit Terrakion and Mamoswine. Hmm, now that I think about it, you can actually drop Forretress completely. I know that sounds crazy, but Forretress just doesn't mesh very well with the offensive nature of this team. Yeah, it supports the team by spinning away hazards (your team isn't crippled by Stealth Rock anyway), but the team doesn't fully support Forretress back by dealing with the handful of things that can set up on it. Bulky stall breakers such as Reuniclus, Conkeldurr, and opposing Latias easily use Forretress as a set-up fodder and all three look like they can sweep your entire team with just one boost under their belt.
To patch up this last hole, I'm going to recommend a more offensive replacement over Forretress, something that can switch-in to these threats and threaten them right off the bat. When you decide to make this a BW2 team, Scarf Genesect > Forretress
will work perfectly, but until then, I think Sub Disable Gengar > Forretress
will be fine. Sub Disable Gengar beats all of the aforementioned threats since they usually only have one attack that can hit Gengar. Once that attack is Disabled, Gengar can just wear them down with Shadow Ball. Sunny conditions make this set even more annoying than it already is by keeping Sandstorm/Hail from negating Gengar's Leftovers.
I know that's a lot of take in, but I hope it was helpful. Good luck with the team. Can't wait to see the BW2 version.