I'm playing my first game and I think I'm doing quite good. I've started as a bohemian duke and I've managed to create Kingdom of Bohemia, expanded a little while HRE had low crown authority, got elected as an Emperor, created a Kingdom of Germany. I lost my Emperor title after death of my first ruler. So now I remain a King of Bohemia and Germany and I managed to expand to Poland a little. The question is - is it possible at all to expand withing the Empire? Crown authority is high at the moment. I can't fight the Emperor's forces now and I can't wage wars again fellow vassals. I can only expand into Poland and/or Hungary - should I do it in hopes I would be able to amass an army large enough to challenge the emperor himself?
When the crown authority prevents you from waging wars with other vassals, there are a few things you can do:
Start a Plot to force your liege to lower crown authority.
You can invite other vassals of your liege to join your plot, although this will depend greatly on how much they like you and how much they like your liege. I often try to grease the wheels by giving them gifts or fulfilling wishes ("wants to marry"). You also could have your chancellor sow distrust, although in my experience that does not succeed often.
Careful manipulation of marriage and murder.
This is normally best done with young adults who are over 16. Find a Duke or Count that has one son (his heir) and a daughter. Arrange a marriage between YOUR heir and THEIR daughter. Once the marriage is nice and locked in, see if you can't knock the only son out of the picture with a well placed assassination plot. It will also be helpful if the target Duke or Count is older in years along with his wife. Keep an eye on his wife from time to time to make sure she is not pregnant.
Sometimes the easiest thing to do is just wait for the current leader to die and the realm to fall into anarchy. Civil war will often break out after an Emperor dies, especially if the heir to the throne is young or if there are many claimants in the family. You can take advantage of the chaos and claim independence, using mercenaries to strike quick blows to force the new liege to yield. This is risky, however, due to the possibility of the Emperor regaining control and bringing about a stable rein. Generally, the more chaos and unhappiness there is within the Empire, the easier it will be to tear pieces away.
Just a few ideas. There are always more ways to get from A to B. I enjoy the challenge of claiming territory within a larger Empire, but you do get a bit stuck when crown laws get to high. In my experience, however, the more they tighten their grip, the more vassals get revolt-happy.
There are a few ways you can expand.
You can marry your heir into the succession of other titles, if they're desperate enough to directly marry off their heirs, or you're clever enough to marry a claimant and then engineer that claimant onto the throne. That will get you new territory when your heir's heir inherits your German and Bohemian titles, assuming they can hold onto their own titles until then.
You can expand militarily to places other than Poland and Hungary. Denmark is right there, as is France, Sweden, Ireland, Novgorod – you are not limited to expanding into territories that you share borders with. Holy War for the Pomeranian and Lithuanian counties is a good option, but beware the dogpile from other rulers with the same religion and the low army sizes the pagan counties can support.
You can found or join a faction to lower Crown Authority, so that you can war on your fellow HRE rulers again. This isn't an offense that your liege can use to justify taking action against you, and if your faction gains enough backers the Emperor will be given an ultimatum to do it or face rebellion.
You can join a Crusade that you know you can win. If you have the highest contribution percentage when your side wins the war, you'll gain all the titles in the region. Remember that occupations count for more warscore than battles do, but make sure the enemy's big armies are taken care of first. Let your fellow Christian rulers scurry around fighting the remaining small armies while you siege and occupy a couple of counties, and your war contribution should be quite high by the end.
You can get elected Emperor again. Curry favour with your fellow HRE electors and get yourself elected to the big chair. Once you're there, either use the power to change the succession laws to something other than Elective (if you think you can keep the HRE rulers happy enough to agree and not rebel), or use the might of the HRE's armies to go claim yourself some new territory that you will distribute amongst your Bohemian and German dukes (or keep for yourself if their succession laws will let your dynasty inherit them) so that you keep them when the election removes your dynasty from the Imperial throne.