A few thoughts and observations:
+ Comparing it to Forgeworld's Crassus*, your carrier is 25 points more expensive, and for those points, your carrier gets better side and rear armor, doubles it's transport capacity, loses the Superheavy rules, loses ~2 hull points (roughly converting
SP to
HP here), and gains 55 Fire Points. The Crassus has a few token armaments, but less firepower than your Carrier with a tooled up Platoon inside. I'd say that your carrier is significantly better than the Crassus.
+ Comparing it to Forgeworld's Gorgon**, your carrier is 150 points cheaper. You've got better armor, but the Gorgon has more effective Hull Points and is also Superheavy. Transport capacities are similar, Gorgon suffers from the Open Topped rule, whereas your tank gains the Fire Points without the vulnerability. Gorgon has a few token armaments, that pale in comparison to the firepower of the embarked platoon. The Gorgon is probably a better vehicle than the tank you've proposed, but yours is much more efficient, having 90% of the utility for 2/3 the price.
How would I change it?
+ I'd consider dropping the armor to 14/13/12. The Guard's heaviest vehicles use that profile, tanks 2x the cost of your transport, such as the Baneblade and Stormlord***, use that profile. 14/14/14 is the domain of the Land Raider, a war machine so advanced it is, by the Emperor's edict, reserved specifically for the Adeptus Astartes. Some
IG superheavies have the option to upgrade the sides to 14, but
AFAIK nothing the Imperium fields save the Land Raider is 14 all around. And as far as the rules are concerned, some armies find it almost impossible to put damage on a 14/14/14. It's not the end of the world with the Land Raider (goal: disembark the troops) but it could be very frustrating in the case of this carrier (Goal: troops stay inside, shooting out at enemies who may or may not be able to return effective fire).
+ I'd add a rule specifically allowing it to carry multiple different squads.
RAW, it can only carry one squad.
+ I'd make it take damage like an Open Topped vehicle. If there's enough room for 55 men to shoot out of it, there's enough room for a Krak Missile to get lucky and fly in through a shooting port. I'd avoid making it truly open topped, though, as having an entire platoon ferried into assault range and come spilling out for an instant assault would be extremely, extremely powerful (and thus necessitate a bigger points bump to balance it).
+ I'd consider making it Superheavy, maybe with 2 Structure Points (equal to about 6
HP).
+ I'd give it a gun. One, because the Imperial Guard stick guns on everything. Two, because as is, it's quite vulnerable to 'weapons destroyed' results, as it has no weapon to take the hit.
+ Depending on how you tweak the other elements of the rules, I'd consider rethinking the cost.
Hope this helps!
* (Crassus) Rules found in Imperial Armour Apocalypse 2nd Edition, but an old version of the Crassus rules can be found here:
http://www.forgeworld.co.uk/Downloads/Product/PDF/c/Crassus.pdf ** (Gorgon) Rules also found in Imperial Armour Apocalypse 2nd Edition, I cannot find any references online in a free and legal format, and I don't have my
FW books on me at the moment so I'm going off memory here so slight mistakes may have been made.
*** (Stormlord) Rules can be found on
GW's website:
http://www.games-workshop.com/MEDIA_CustomProductCatalog/m2440066_Imperial_Guard_Datasheet_-_Stormlord.pdf