Yeah, it seems as if each series after 3 or 4 was written believing it would be the last. I guess they had to plan for that, though. MC is on record talking about purposely not resolving the M&L relationship issues for fear of the show becoming boring. So every series they have to start again, and yes, after 2 or 3 seasons of the back and forth it becomes a bit tedious.
Personally I think they should focus less on M&L and more on the other characters. IMO some of the best episodes were more centered on other characters... Mark's stag party, Bert's restaurant, Terry Glasson's heist, Aunt Joan's lawsuit. All of this with M&L interacting with the other characters while dealing with their own issues.
That is, don't focus so much on a tortured relationship. There's probably still a lot of fertile ground in the other characters' story lines. If the writers can just get M&L back to that place where they were a little more comfortable and open, not as paranoid about each other, and engaged with the villagers, maybe the joy that comes across in the first couple of series can be rekindled.
About S8, I suppose there won't be an official announcement until next year?