Well, I do wish they'd have more gold available for bartering, but that might unbalance the game. In vanilla Oblivion, merchants had unlimited gold but I don't know which idea is better.
They only had unlimited gold
in total. They were, however, limited in how much they'd spend per transaction (making it virtually impossible to get full value from high level, enchanted gear -- even with Speechcraft maxed and investing in the shop).
Haggling/bargaining would be nice (I haven't found an option for this at least?).
Well, in Skyrim, I think haggling/bargaining is
implicitly there and is reflected by your speech skill and some of the perks in that tree.
I don't know about level-scaling items. I kind of like it the way it is, but maybe that's because I'm lazy.
I don't like the auto level-scaling, as others have said, because it's too player-centric and, therefore, an immersion breaker. It'd be much better if different regions specialized to reflect regional resources and if the local economy played a role.
A quick example of both: Markarth should obviously be the region for dwarven gear and, further, the more bandits/forsworn the character kills in that area, the more prices should drop since supply should increase due to safer trade routes. In this manner, it'd make it harder for characters to spam kills for loot in one area without seeing diminishing returns to sell in the same area. Otoh, the more bandits/forsworn killed in the area, it should become more likely for dwarven materials and gear to show up in the inventories of merchants in other regions.
And just an aside, but it'd be really, really nice from an immersion standpoint if merchants recognized the character as a frequent, repeat customer. As it is now, you're treated like any random, new customer by almost every merchant unless you've done a quest for them -- even then some act like they've never seen you before.