Give this man a cookie. I personally think this idea is perfect. I don't think it would be too bad on the server more of thing to your PC. I maybe there could be let's say on the world map there might be locations of important battles ie the siege of the ork stronghold by the strike force of for ultramar was broken by the orkish strike force of 'ead krumpas. This would also completely and so much more accomplishment when we inevitably go back to that contienit and see the huge crossed swords on the map. This could also easily create strike force rivalries between factions

expanding on that train of thought, you could implement a sorta form of persistense: let's say there's a challenge, and the challenge is met by a strikeforce (groupchallenge) you get a permanent graffiti marker on a wall of a bunker or a tattered standard.
when the war for the planet is complete and resets, these markers remain visible, and if you go with your mouse over it you see the name of the winning group, date and achiev.

I'd personally go for a battered torn up banner, cos immersion wise it wont look out of context when you come across it.

Keep in mind that the Devs have said we will basically be fighting for continents not the whole planet, once one continental campaign is over we move to the next continent,
So each continent will have its own predominate landscape/structures/weather,

The ideas could still be implemented having battle markers/memorials etc remain behind,
But also the "flavour" of things such as structures/buildings etc could be shifted towards the faction that won the continental campaign,
Meaning next time we cycled back to that continent it would look different having been remodeled to suit the previous campaigns Faction victors.