Could miners create a cartel to raise transaction fees? Could miners with enough mining powers (say >95% of all blocks) form a cartel where they demand absurdly high transaction fees? Of course, I still could could hope for a miner not in that cartel to relay my transaction for a lower fee, but in that case I couldn't be sure when my transaction gets paid to the adressee.

Is this a plausible scenario?
Miners have huge investments in hardware that can only be used to mine Bitcoins. They have no incentive to do things that reduce the value and usability of Bitcoins. Any "mining cartel" with the power to get 95% of the hashing power to take collective action in non-emergency circumstances would create enough fears about 51% attacks and the like that it would drastically reduce the value of the Bitcoin system and greatly encourage competitors. So while this is theoretically possible, it's unlikely in the extreme.