Welcome to the forum.
As you're new to making fan cards, I'll give you some tips I've found useful myself to making compelling ideas.
- Firstly, try to think of things that the current Dominion cards don't do, things that initially seem interesting to you. Likely you've done this. I like to aim to be different; if an idea isn't that innovative, it likely won't add much to the game. But there's nothing wrong with being similar.
- Check that the idea isn't broken in some way, either in functionality or in being overpowered. Think about all the implications that it will have, and check that no official card comes to any problems with it. This can be hard, and most likely that's how this forum will help. At least, it will give you direction for playtesting, the bottom bullet point. In time, you'll get used to principles to follow. There are reasons official cards don't do certain things you'll get familiar with*.
- Try to express your ideas as if they were written on official cards. The simpler you can explain them, the easier it will be to sort out the rules for them, to continue with the other bullet points here and to get better feedback from this forum. It's best if you use phrases that already exist on official cards.
- And the important but fun bit, playtesting. Check how the card plays, run it against official cards you think it could be strong with, and also feel how interesting it is to play. You might be able to make it more exciting, or you need to simplify it; I've learned myself that simpler is better. If you have a play group your fan cards will be played with, you may want to do a bit of solo testing first, to make clear any immediate problems so your group doesn't have to suffer through them.
Loosely speaking you can do these points in order, but you'll likely go back and forth to get an idea good. So, let's compare your Sultan:
It's definitely different. Probably the idea excites you.
It changes vanilla bonuses, so the stronger implications of it will be with cards that give lots of vanilla. It makes them very flexible. You'd run through some of these and see what would be strong. Examples that come to mind as well as those already posted are Wine Merchant becoming +4 Cards +1 Action, and (Grand) Market +3 (+4) Cards +1 Action or some on Coins.
Sultan is so different that wording could be tricky. Types, for instance, are the words on the bottom of a card, like 'Attack'. Teacher uses the word 'bonus' for its tokens, so this is the term you'd likely use. The wording at present implies you choose vanilla that other players get outside of turn, as majiponi pointed out with Council Room. This can bring you back to the broken test; I'll add Caravan Guard.
Playtesting, though, I imagine will be very telling about the quality of Sultan. As well as thinking through all the things identified above and the other posts here, I don't think it would be a pleasant play experience. There's just so much extra decision making, and a fair bit of mental pressure as you choose whether +Card or +Coin is best in your current situation, what's still in your deck. There's also the issue of tracking what you chose for each card, as Holunder9 brought out. So, a principle you can draw from this is that it's good for cards to be rigid. If you make cards flexible (like Steward or Count), make sure they don't overwhelm.
And you mention about making a set. It seems strange to make a set about 'card interaction'. Really, the whole game of Dominion is about card interaction. Do you perhaps mean cards that adjust what others do, like how Sultan changes the vanilla of other cards? Hopefully the principles of ease of tracking and play experience can help. It could be said that Village interacts with Smithy to change what it does; it means Smithy can draw Action cards and they be usable. If there is no Village, it can only draw Treasure or Night cards. And Throne Room is entirely dependent on interaction.
*Another great help is the stickied post, the guide to fan card creation by rinkworks.
A lot of stuff here, hope something helps.