What do you do and get once you are Ingress level 8?
They wait until new part of gameplay from Niantic :
- missions (not only reserved to the tutorial part)
- achievements and badges (à-la-stackexchange ;) ) (edit 28/08/2013 : in progress :) )
- new items
- new levels (9 to 16)
- new strategies due to Regional Scores at 5 hourly checkpoints
- ...
source
Apart this teasing, for the moment when reaching the level 8, you gain the ability to use the mass destruction weapon that are L8 XMP Bursters (2700 damage around 168 meters) ! No portal can resist to this (a full shielded L8 portal can be destroyed with 4 XMP L8). But the levelling up driving force is dead, you'll have to find new motivations.
To have such powerful items as L8 XMP, you'll have to build powerful portals (at least L6/L7), and to build such powerful portals, you'll need many powerful teammates: you need to be 4 L6 to make a L6 portal, you need to be 6 L7/L8 to build a L7 portal and 8 L8 to build a L8 portal.
So, one good way to use this power is to help your teammates to level up. For example, a L8 player can destroy portals (or take them down to 1% power), while lower levels finish the job and rebuild the portals to gain AP.
The hardest thing to do in the game right now, which takes cooperation from 8 people, is making L8 portals. These are the only reliable source of L8 resonators, outside of zip code bounties and rare drops from L6 or L7 portals.
Once you reach L8, it becomes more of a mentoring game where you try to help your side level up effectively - after all, a single person can't do much and you need teamwork to fight off assaults from the other faction.
As you can realize, both of the above are social aspects and not really desirable by the 'lone wolf' type of player, those tend to stop once they reach L8 (though its likely they will return once more levels are introduced).
Once you lost your sense of living, i.e. collecting AP for L8, you indeed have to search for new objectives.
If you are playing in a larger town with an active community, cooperative missions like raids on neighbouring towns, very large fields, artwork on the intel map or L8 farms can be a lot of fun if you are the social player type.
One noteworthy objective we saw in my hometown was to colour the town WHITE, which requires yet different alliances.
Chasing these new objectives becomes a lot easier if you have access to the most powerful of weapons, also being L8 separates you from the 'ordinary foot soldier'
"It is a ticket to another world" (Lucas Arts, Monkey Island II)