I hope there'll be at least one useful vendor-bought equipment.

Oh and having fast travel cost money would be immersive and a gold-sink

I would have shopkeepers only buy sensible things. He wouldn't buy 15 elven cuirasses from you, simply because there isn't that much demand for them. Instead he would buy one or two of each item type you have, and then he wouldn't buy that item type again for a few days until he has 'run out' and needs more.

Different areas could have higher or lower demands for certain things, like a dangerous area would have a higher demand for armor and weaponry, but less of a demand for jewelry. You would have to find a wealthier area to sell jewelry.

Something that you can do continuously to spend money for a very without it being boring. For example, being able to run a shop and you have to spend money for goods. Or maybe running a settlement. Eventually that place might grow into a city. Or spend money repairing dragon attacked cities, although after the main quest this wouldn't really work.

agreed. i want a more balanced economy in skyrim.

Eventually, when you are level 45 or so, you will be rich.
This I think is natural. I dont think an RPG where at endlevel you are struggling for money is a good thing.
It would take away a sense of accomplishment.

Good things would be what Daggerfall had. Ships, houses etc. That could cost 100.000's of septims.
Things to spend your money on at endlevel. I dont know. Buy a mine, upgrade it, get some profits, deal with corruption/raiders.
But there should be enough room to feel like scrooge mcduck.

Stuff I want to be able to buy in Skyrim when I have way too much money:

Houses:
These are all great. And the ability to "upgrade" them with newer furniture and decor is nice too. Further enhancements are a must, like multiple levels of furniture and decor quality, instead of just one.

Business Investments/Possible Returns:
Invest a sum into your local business and watch it thrive, then collect your return investment. Or watch it sink like a granite rowboat and weep bitterly on the curb in the rain.

Buy the business outright:
Those who work there are now your employees. Either let them run the shop their way, and collect the profits, or make changes for better, or worse.

Ships:
Use it as a floating base of operations. Lease it out to traders who own no ship of their own. Buy a whole fleet and start running your own East India Company style trading route!

Assassinations:
The services of the Dark Brotherhood do not come cheap, but if you want someone dead and don't want their blood on your hands personally, it's totally worth it.

Personal Estate:
Start by purchasing a plot of wild, untamed land. Choose strategically for optimal access and trade possibilities. Hire a team of carpenters and masons, choose what kind of building to build, then set them to work building it. Hire further people to work the fields, the smithy, the kitchens, and guard duty. Take the crops, weapons, equipment, and other goods and set up a trade route.

Beasts of Burden:
Like horses and mules. It takes money to maintain them too, so you have to pay for stabling, feed, etc.

Charity:
Share your wealth with the poor communities of Skyrim and watch them use it to rebuild themselves a better life! Or watch them squander it on booze and skooma.

Research and Development:
The guild of mages have some great ideas for some new kinds of spells, but they don't have much funding to make it all happen. Help them out financially, and they'll repay you with magic or magical items nobody else has!

Maybe you can buy a small village or a castle somewhere that you can continually upgrade in size, furniture, features, weaponry, and guards/soldiers.

You can then send out raiding parties to other villages/castles and in turn defend your own, in which you can pay for repairs from any damage caused (thus never running out of things to spend money on).

I like that I am always an archer of some sort so the dagger and bow upgrades have me very happy.

Well, taxes are defined by being a pain in the..., aren't they? But there could be more in it. They could be optimal - something you have to suffer if you want to live inside a city for example. Or you could contribute with more than you needed and raise your reputation in the society that way. Death and taxes are unavoidable in RL. In TES there's plenty of death, but no taxes. It should be better balanced.

Weighted gold would be an interesting idea...at the beginning it wouldn't be a big deal, but eventually you'd have to find a nice safe cubby hole so you could walk around. And add the possibility or npc robbers...and then the quest of finding those no good idiots...banks could be thrown in there as a place to keep your money safe...and if your a theif you could rob said bank if you were crazy good at sneek...the possibilities keep coming!

Anyway, increasing buying prices while decreasing selling would be a good start at least.

typical problem in games... but the worst scenario must be those from Assassin's Creed II

I'm not convinced that really works. It can start to fell like a grind trying to save for something or like the game is ripping you off because of the discrepancy between sell and buy. I can't think of a game that has done the economy side of things well though.

Either gold needs to be far less common or they need to greatly increase the cost of the items in shops and the amount it takes to buy a house.

Preferably I'd like to see gold just be less common

I have zero problem with being able to become wealthy at end game.