This is just for fun, not considering balance. Obviously just don't make the ability directly more powerful, actually think about the concept, dont just say "id make backstab do 10 times more damage unless you've got some good reasoning.
Also consider stuff like a glyph, where you have to give something to gain something, and talents too, or spec feel overall. Also, are there any abilities you'd get rid of?
This is just for fun really, I miss my rogue but sometime it just doesn't feel stealthy enough.
Here's some ideas:
Evasion,
It just seems a bit lame now, sure it can be useful but it's not necessarily interesting.
I'd consider adding a stealth to evasion or some kind of invisibility but not actual stealth. You know like with subterfuge? I'd make it some kind of 15 second subterfuge, not necessarily "stealth" but just some kind of invisibility or sneakiness. Could be interesting in pvp, to allow you to attack players and not be seen. It could obscure the target like smoke bomb does but that might be overpowered.
Garrot,
I started off with this idea as a passive but it sounded over powered, I kind of stole it from the ninja perk in call of duty.
What if whilst a target had garrott applied to them, they couldn't make any actual sound? So they can't aggro/call for help, and the sound of your attacks cannot aggro another NPC or, more interestingly, another player? You wouldn't be able to hear your teammate being attacked, you'd have to spot him, which might be difficult to do if he's stealthed. Sound's kind of sinister, right?
Slice and Dice
I'd change it into an active ability. I sort of like the idea of having slice and dice being channeled and rapidly building additional combo points, so you use it when your energy is empty and a few seconds later you've got 5 combo points. If slice and dice got removed, it would lower our passive damage, giving us a change to deal more active damage (as was one of the promises of cataclysm which never came).
I'd sort of energy regeneration by significantly reducing energy regen bonuses from talents/specializations and instead make abilities cheaper. I'd also get rid of the haste increase energy thing - it sounded nice at the time but it's holding us back in other ways. I'd basically make us do less passive damage by slowing down our attacks, which would allow our active abilities to deal more damage.