About Rental: Originally it shuffeled the two Coppers
and the Rental itself in your deck, but i couldn't find a wording that does that (loose track rule and all...). I also like engines more than BM, but i think so many card support Engines it was time to add something different.
After a quick read-through, the three biggest issues that I see are:
Nouveau Riche:
This is a really weak card. It's way worse than baron. $4 and a buy is clearly better than 3 cards or $2. You need a stronger effect than 3 cards, I can get that for cheaper on a smithy. And it's reliable! $2 is even weaker than 3 cards, and +1 action is useless.
So, I would up the power level of the effects. Maybe +4 cards and +$3, +1 action would be good.
Parliament: Pretty much worse than throne room; I would rather play my witch or market twice than my oracle three times. I could be wrong there though. I was going to say that it's OP if highway or bridge is on the board, but if you can play 5 highways in one turn you are probably going to win soon anyway.
Rental: I don't like this because I sense that it will just be a better province in many games. It discourages engines as well, and generally people prefer engines to big money.
I removed Nouveau Riche because i felt that too many cards seem to scare reviewers (do they?), but i'd like to mention i actually don't have a clue at which price Nouveau Riche would be balanced. I felt it was okay when i playtested it for 4$, but i'm a mediocre player that often tries strategies that are far too complicated. I'll work on it, again.
About Parliament: Probably also needs some working. Would 4$ be too less? I was thinking of a 5-2 split, but probably it would be more interesting to early KC a 3$ with this...
Rental
7$, Victory
Worth one per Coin Token on your Rental Mat at the end of the game.
When you gain this, put it on your deck and gain two Coppers.
Setup: Each player gets 7 Coin Tokens and places them on his Rental Mat. Whenever a player trashes a Copper, he must instantly remove a Coin Token from his Rental Mat.
How different would the play be if you just made the VP value of Rental 2 less than the number of Copper in your deck, with a minimum value of 0 and a maximum of 7? Less messing around with tokens.
Interesting idea... But i wanted to keep players from trashing
any Coppers. So you could just buy them at the end of the game, as eHalcyon pointed out. By the way, the idea for Rental came up when we discussed Copper replacements that are worth VP, so i'd like it to keep that "don't loose your Copper"-aspect.