How about some weirder, sillier requirements along the way?
* Destroy 100,000 barrels.
* Kill 1 Baron...with your fists.
* Kill 1 Archvile...with the combat knife.
* Kill 1 Cyberdemon...with the chainsaw.
* Kill 7 enemies with one shot.
* Use 3,000 levers.
* Reach level 19 in 10 games.
* Reach level 20.
...and so on.
Destroying 100.000 barrels, use 3.000 levers, and other requirements not related with KILLING: If you haven't realized, all the rank requirements are set around killing different amounts of different kinds of enemies, in some way or another, so these requirements don't fit.
Kill 7 enemies with one shot: Oh, too easy! Even I can kill 7 enemies in a single shot! By the time I reach the level with the Lost Soul rush, I already have gotten the first BFG9K.
Kill 1 Baron with your fists, kill 1 arch-vile with the combat knife, kill Cybie with the chainsaw: Now we're talking. Although, let me tell you, someone who has managed to kill the Angel of Death in the unholy cathedral will already have enough training to attain these challenges more or less easily. The Angel has a shit-ton of HP, more than Cybie, AND you have to take him out in melee in order to retrieve the Spear of Destiny. Therefore, we need some flavour of this requirement that proves to be a challenge, but still is out of the threshold of absurdity.