Who knows? The system has not been fielded yet, and may never be. It's kind of hard to tell with a what-if sort of system, especially the FCS which is a whole family of vehicles and whose protection level, etc., are still debatable and notional.

Certainly eventually there will be something like that. Problably not the FCS specificly. The old LVT 5 had a 105mm gun version that was primarily intended for direct fire. But, was used ocasionally in the indirect fire role. So the Marines have fit that sort of thing into the tool kit before.

Does anyone know if it is possible to construct a family of vehicles based around the AAAV? I'm thinking of heavy mortar carriers, light tanks, ATGW vehicles etc?? Would fitting a heavy turret to the AAAV mean that it could no longer swim to shore?

There are some serious engineering challenges in fitting a large gun. The older LVT models had lower centers of gravity. Plus the thing is aluminum. Not the first choice for carrying a heavy assualt weapon. A mortar might be viable. The experimental EFV probably cant be converted either. With the power plant in the center it looks to my poorly trained eye to be impossible to mount any sort of turret. I'd been told by Marines who werre around then, that in the late 1960s it was determined from experince that bringing along a platoon of main battle tanks was more efficient than trying to convert a batch of underarmored infantry carriers into a assualt gun carrier.

The current AAV/LVTP-7 has a couple of variants and mods-Command, Recovery and some have been modified carry a MICLIC I belive. This vehicle has just about reached the end of its useful life. Its suspension is overload by added armor and it is just palin wore out. Other than possible Command and Recovery types I do not see a lot of other variants for the AAAV/EFV coming down the line for the AAV/EFV. They are not buying enough of them to replace the AAV-7 on a one for one basis as it is. If you take a more specialized versions from this number you even furhter decrease available lift assets. Remember the primary function of this vehicle is not as an IFV but has a ship to shore lift asset.

"...not as an IFV but has a ship to shore lift asset." Make that 'Amphibious lift asset. They are great for river crossings, flooded and marahy ground. Roeblings original 'Alligator' vehical was designed in the 1930s specificly for navigating hurricane flooded ground as a rescue vehical. Ship to shore is a subset of the ways these vehicals can be used.

with the exception of the new aav that will perhaps make its debut by 2020 or 2050...whichever-and I'm thinking that it will compete with the osprey for the longest devolpement project for the corps yet the marines have had various assault guns in the past, some good, some bad-notably the ontoes(sp)..a few others were conversions, but stemmed from the philosophy of ww2 and korea when asgs were still the thing..... the marine corps ever shrinking pitiful budget will not be able to absorb another vehicle..... the only thing I can see happening is the futuer aav will have the power and transmission/suspension to handle a large calibre weapon as needed. But I see them happy with what they have. Their doctrine support it well and dont call for another av..... the only thing I see them doing, and they have done this in the past is doing something with the lav-25 the now have the lav-25,(401)lav-m with a 81mm mortar(50)due to be replaced by the lav-efss-a 120mm mortar to be bought by the end of the yr lav-at-tow equipted(95) lav-c2-command and control(50) lav-ad-(17) for air defense plus a recovery version in 1990, they cancelled funding for a 105mm armed lav asg so, I dont think they was the asg, just the mortar firesupport at the most... remember, a key is that it has to be transported by the ch-53e and no more than 32000lbs(16tns) if they tried anything else, then it would have to come off the new aav, but I doubt it

I took a quick review of some items writen by the Marines who fought in OIF & Falluja and have the impression that an assualt gun version of the AAV or the LAV would be inferior to the Abrams tank for infantry assualt support. Modern AT weapons are such that an assualt gun vehical needs to have tank class armor. With new types of HE ammo for the Abrams you get the armored assualt gun, and a MBT in one package. Not perfect, but still very good.