We need to make two changes to MMP to get the corruption out of it.
First, as this blog has already said, if you got the threshold in the previous election you must stand a candidate in every seat. You may not drop an electorate to do a deal with some splinter Party to get advantage for your party and it.
Second, any party that does not get to the MMP threshold but also wins one of more electorate seats should get no more than that/those seats, not the percentage of seats represented by its under-threshold party-vote.You do not get the under-threshold percentage of seats.
That will eliminate the corrupt trick so beloved by the National Party of letting an under-threshold party ride in on the coat-tails of a lone ranger, then the lone rangers' party exaggerates the big party's coalition.