A few months back I proposed some changes to the current badge groups.
Here's the post: it's big but covers everything pretty extensively.
As far as new badges go, I mention a Gatekeeper Gold badge (Clear UV Hellgate), which could be expanded to a series.
Gatekeeper Bronze: Defeat the Bruiser Brothers
Gatekeeper Silver: Exit Hellgate alive on Ultra-violence
Gatekeeper Platinum: Defeat the Bruiser Brothers on Nightmare!
(Luckily, BB count CAN be measured given that there's only two in a game.)
Although it doesn't seem that practical, running/phasing through Hellgate is a really easy way to get through it. What makes killing the Bruisers worth a Plat on N! is that the first wave is hard to get rid of, and the Imps will just open the door if you phase past (not to mention the Bruisers are gonna be fast). Unfortunately there's no real Diamond equivalent unless you wanna add, say, AoMs or AoMr/AoSg.
Including the Compet-n medals in some badges would be cool. For example:
Racer Bronze: Beat standard game in under an hour
Racer Bronze: Beat Angel of Pacifism with Compet-n Silver
Racer Gold: Beat standard game with Compet-n Silver
Racer Platinum: Beat Angel of Pacifism with Compet-n Platinum
Racer Diamond: Beat standard game with Compet-n Platinum
The last one sounds nearly impossible, actually. In spite of playing on even ITYTD, you'll need either sufficient rockets or the BFG (that is, complete HA or HoC or carefully get it from CoS) to down Cybie. (Alternatively there's a zerker route, but that takes leveling and leveling takes time). In any case, the luck and experience needed is at least of Diamond level, so I think it's fair. Changing Gold/Diamond to HNTR might make more sense, but speed is hard all by itself and the difference between ITYTD and HNTR probably isn't enough to make a big deal.
Some medals:
Firefighter's Trophy: Clear Phobos Arena without taking damage (probably can do a check before/after level on HP for this)
Personal Mortar: Move at least fifteen spaces with a single rocket jump
Suicidal Hopper: Make at least twenty rocket jumps in a single game
Dead Heart Medal: Die with at least 100% health within one second (great way to keep track of how many times you've been screwed)