My overall advice to you: Quality over quantity. Look for the unique essence of each card that you create, and do any revisions necessary to bring it out, instead of trying to give a name and a cost to every gimmick that you can think of.
Telescope $2
Action - Reaction
+1 Action
Look at the top 5 cards of your deck, then put them back in any order.
______________________________
When another player plays an Attack card, you may reveal this from your hand. If you do, draw until you have 8 cards in hand, then put cards from your hand on top of your deck until you have 5 cards in hand.
A majority of the time, this won't make any net effect, even though sometimes one might use it to avoid dead draws of actions. At least Navigator gives coin and the option to discard all the revealed cards. The reaction is also problematic. Giving the player 8 cards to choose from instead of 7, this reaction is strictly better than Secret Chamber's in the vast majority of cases (barring previous plays of Governor or Council Room). What's more, it heavily penalizes any non-hand reducing attacks. Especially terrible for the attacker would be playing a non-hand reducing attack when the attackee has less than 5 cards, giving them a free restoration of their hand, picked out of 8 cards (which is almost like a free play of Warehouse, minus the discarding aspect). It would definitely skew the game too much.
Drunkard $2
Action - Attack
Each player (including you) reveals the bottom card of his deck and either discards it or puts it on top, your choice.
This is only negligibly different from Spy, barring the lack of +1 card, +1 action. Spy itself is a very underwhelming card with a weak attack and a low chance of being beneficial. It's not a card that needs a copy, much less a cheaper, terminal imitation that becomes more of a liability to your deck than anything else. Even if it costed $0 I would ignore it.
Luxury Tax $2
Action
+$2
Trash this. Put a Tax token on top of a Supply pile.
Each card cost $1 more per Tax token on its pile.
This would have interesting interactions with trash-for-benefit cards, but overall, I think this card is fundamentally flawed. It's liable to make games become unbearably drawn out, and can very easily make them almost unfinishable. Just three plays of this could make Provinces cost the same as Colonies, permanently, which wouldn't be very fun in a game without Platinum or other Prosperity cards. At least with Embargo, you're running out the Curses, which pushes the game closer to finishing. And with trashers on the board, people won't terribly mind buying something with 1 or 2 Embargo tokens on them. Plus there are workarounds when you gain without buying.
Aerial Crank $3
Action - Reaction
Choose one: +3 Actions; or +1 Action and +2 Buys.
When you discard this from play, you may put it anywhere in your deck.
______________________________
When you discard this other than during a Clean-Up phase, you may reveal it. If you do, set this aside, then at the start of your next turn, put it in your hand.
You probably have reasoned that, since this card is only giving actions or buys, it's a harmless enough card to make so sticky, able to play multiple times per reshuffle. I would agree if this were an alternative Prize, since we'd only be dealing with one copy, but things could get very screwy very quickly when there are multiples in a deck. This is just too easily abused by discard-for-benefit and discarding-as-the-mitigator cards. Also, this would need a differently-colored back like Stash has, since its location is a publicly-available datum that shouldn't be obscured by bad memory. You might be able to salvage the reaction for something else, however.
Saloon $3
Action
+$1
Discard any number of Action cards from your hand. +$2 per Action card discarded.
Overpowered without a doubt. And it essentially turns actions into silvers, which makes for a boring strategy not much different from Big Money.
Gate Watcher $3
Action - Reaction
+1 Action
You may reveal a card from your hand. If you do, return it to the Supply.
Each other player reveals his hand. For each Attack card revealed, +1 Card.
______________________________
When another player plays an Attack card, you may reveal this. If you do, you may reveal a card from your hand. Return the revealed card to the Supply or put it on the top or bottom of your deck.
This shouldn't be so dependent on Attacks for its main effect. Plus, the two components of the main text are just too disjointed and may as well be separate cards, since together they don't create any overall effect. And returning the the supply is a bad idea that will only slow the game down. Ambassador gets away with it because it distributes those very same returned cards to everyone else.
Runaway Train $3
Action - Attack
Each other player discards down to 3 cards in hand. For each card he discarded from hand, he discards that many cards from the top of his deck.
You shouldn't be able to open with the equivalent of two Militias. That would ruin any game from the start. And since it gives no benefit to the attacker, it will only make games long, drawn-out, and not very fun. (eg: You've been Runaway Trained so your hand is RT, copper, copper. You play RT on your opponent so their hand is RT, copper, copper. Rinse and repeat). Sea Hag costs $4 so it can't be opened double, and it eventually loses firepower when curses run out. The free deck-cycling that this gives to opponents doesn't have any purpose, either.
Scoundrels $3
Action - Attack
Reveal a card from your hand. Each other player discards a copy of that card from his hand (or reveals a hand with no copy of that card).
You shouldn't be able to cockblock powerful cards so easily. Cutpurse only targets coppers, and handsize-reducing attacks let players choose to keep their best cards. And even if you Masquerade someone's 3-card hand, they still get to choose what they give you, and you have to give them something in return. Also, this card would make the last player in multiplayer stand no chance.
Prospector $4
Action - Attack
Each other player reveals the top 2 cards of his deck. If they revealed any any Treasure cards, you play one of them of your choice. They discard the other revealed cards. Any treasure cards you play this way are set aside and returned to their owners’ discard piles at end of turn.
I would tighten up the wording here. And this is definitely not an attack (see Tribute). And unless you're playing multiplayer, or everyone's flush with Gold and/or Platinum, I don't think this is good enough to be a terminal $4, as the best you'd hope for is a silver.
Outlaw $4
Action - Reaction
While this is in play, when you would gain a card, you may immediately discard this. If you do, each other player gains a copy of that card instead.
______________________________
When another player uses a Reaction card, you may discard this. If you do, that Reaction card is immediately discarded and has no effect.
First of all, I would say "activates a reaction", for when someone uses the reaction mechanic of a Reaction card. Second of all, this is a terrible idea. Punishing a player for activating a reaction by first nullifying it and then (figuratively) slapping it out of their hand won't make the game any funner, it will only make people refuse to buy even interesting reactions like Tunnel, Fool's Gold, or Trader. Reactions which nullify the harm of Attacks will not completely disincentivize the use of attacks since the Attack usually has some benefit for the attacker. Even if there's a strong attack on the board, Moat is not always a wise choice, especially since you harm your deck if you load up such a weak action. Trying to cockblock it with something like this is overkill. And the ability to take someone's Watchtower away when they use it to top-deck a card would be awfully.
Well, analyzing those 10 cards have left me a bit drained; I'll come back to the rest if I feel like it.