Marrying heirs is so easy to exploit. I goofed around with it a bit and as an Irish count I managed to inherit Scotland, England, France, and Denmark within 50 years and during my first ruler's life span. He had a ton of daughters, so I just married his daughters matrilineally to sons of the rulers (if you invite a second son to court, they'll usually come due to being pretenders, then you can marry them however you want) and started plotting to kill everyone in the way and waiting for a grandson. As soon as a grandson was born you set it up so that the son in law inherits, then off him, then off the grandson, and bam - title is back to you.
In the case of Denmark the ruler was female so my heir married her, fathered a son, then I had her killed. The funny thing is the son was the spymaster, so when I plotted to kill the wife he joined in without any persuasion and gave +110% to the plot score. Then after she died I plotted to kill the grandson to give myself the throne, and the son (still the spymaster) joined in the plot again. So I basically just sent a note to my heir saying to kill his wife and son and he's all "Ok!" and a few weeks later I'm king of Denmark and I gave the son all the non-king Denmark titles (two duchies and 5 counties). Denmark was elective, but I was still counted as the top heir when it came to it.
If I had been more active I could have probably taken other kingdoms all over the place, but it was mostly a test run. Might even be able to nab the HRE like this, although you may have to murder more people to get your grandkid and then yourself in as the favorite for the electors.