1) Have the Thieves Guild Npc's attack on sight anyone without a certain level of infamy. Make your nefarious choices count, and also give people who are KOS in most cities a place to hang out.
2) Would love to see the shadowmarks get a more prominent role, good call there. I always wished they had given them more importance in Skyrim.
3) Probably more of an Assassins guild one, but have the ability to post a bounty on a player in Cyrodill, claimable by anyone. I'm sure some would argue it would lead to griefing, but hey its pvp, and it needs to be tied in with the pve side imo.
4)Furthermore, I'm hoping players can visit Evergloam, the realm of Oblivion that is ruled by Nocturnal. It is the realm of perpetual twilight.
Make it the default RP zone, I'm sure it will be filled with vampires and werewolves in no time.