Term limits, though. :)
In practise, most german presidents will just be way too old four years after they leave office to be a viable candidate, and it looks a bit absurd for them to be pushed back into the office, exactly because there's very little real power.
I agree though, it's not an issue, precisely because the German president has historically not acted partisan (even though he's usually a member of a party) and not be involved in day-to-day politics, so it's not too hard to find somebody that everybody can agree on. This may change if the office becomes more politicized, but I doubt we'll see somebody get re-elected after having paused for a term.