Would they even be able to fire the Titan missiles from their main guns? I'm not sure if the "Titan" was designed for that, it's possible, but it seems strange they would use the same model of missile for use on anti-air vehicles, APCs, in aircraft and in tanks as well...

The "Slammer" is based on the Merkava which can fire the LAHAT ATGM, the T-100 probably fires something similar like the Svir/Refleks.

In any case they should be able to fire some kind of missile from their main guns, if the current Merkava and T-72, T-80, T-90 can do it I somehow doubt the future ones would be unable to.

or you could do this which is what I do
this addweapon "missiles_SCALPEL"; this addmagazineturret ["2Rnd_LG_scalpel,[0]];
now if you want more you can either repeat addmagazineturret command or make it instead of 2rnd to 6rnd or 8rnd. note you can only! have 2rnd, 6rnd and 8rnd in one magazine! hope this helps

^that's useful for solo play, etc, but for the average "vanilla" online scenarios we will have to wait for an official gun-launched missile to be added.

And by the way, the LAHAT missile on the current-day Merkava can be fired indirectly - someone else can designate the target, and the missile can be fired from complete cover and at ranges up to 8 km.

Titan would be a standin, really. It´s available, everyone uses it, these tanks need more diverse weaponery. The slammer especially needs to be expanded as far as weaponery goes. Irl it´s an absolute monster, it should be ingame too.