Oh, and also alien subs can only be breached by the TftD Blaster Launcher equivalent, but starting XCom crafts can be breached by alien grenades/sonic cannons. And aliens know that XCom are in the sub, so they do target it with grenades to open it.
Yes! this would be great!
Make external USO doors (only the doors) breakable by standard high-explosive.
Also make Aliens target the Triton, with the same mechanics used for destroying X-Com facilities in base attacks (no idea how it work, coded in the maps?).
They can use grenades and blasters only from turn 3, according to difficulty.rul, so, after landing, you must deploy and take cover, quick quick quick.