Endgame for both sides has been discussed. In a nutshell it involves the sacking of Dromund Kaas or Coruscant - and the success or failure of such a move depends on how successful either side is in the war. If the GE are winning, they will not be ambushed on Dromund Kaas by Harrowers (it will also be easier for them to discover the coordinates) while they are looking for the Emperor's invisible fortress. Which they can blast to pieces unhindered. They will also need a sizable fleet to invade the planet and overcome the initial forces which depends on advantages etc etc
And the same goes for Coruscant. They will need to be winning the war so the GE forces are spread thin, so they can easily blast a whole through the SDs guarding the planet and orbitally bombard the imperial palace. If they are not winning the war they'd likely have to contend with a SSD and reinforcments which would no doubt be lying in ambush (due to Sidious' foresight). But if they are winning the war the intial force will be inconsequential as will the reinforcments which will be minimal. And if they are winning the Ascendant Spear will likely be still operational and will strike faster than expected, allowing them to bombard the palace (and kill the Emperor) before reinforcements can arrive. Then there's the chance Sidious could escape, either via the Lusankya (unlikely) or his personal Sith Interdictor (likely). The success or failure of such a move also depending on the status of the war (not as much though) as the SE would have stolen the plans, or simply stolen, one or several interdictors as it was technology they did not possess. Much like Angral stole the plans to various super weapons.
So as you can see both combatants are perfectly capable of being killed, it just depends on who wins the war.
Just throwing it out there, but the Ascendant Spear's surprise advance strike on Coruscant wouldn't work, what with the planetary shielding and Golan battlestations and whatnot.