I like the concept a lot. One thing to watch out for with these is the risk of making it too easy to pile cards. If Potter's bonus wasn't limited to Silver like it is, then with 1 Route token you could pile the Potters (gain one, using the Route to put it in your hand, then getting a Route and an Action and repeating), which I presume is why that restriction is there (or maybe it's just to make it like Sculptor).
As it is currently designed, I do believe it is possible to pile Low Road together with Villa after playing a Low Road, although it would cost $10. You would:
Play a Low Road
Buy a Low Road ($3, $7 left), using the Route token to put it in your hand
Buy a Villa ($3, $4 left), putting it in your hand and returning to your Action phase
Play the Villa (+$1, $5 total)
Play the Low Road
Buy a Low Road ($2, $3 left), using the Route token to put it in your hand
Buy a Villa ($2, $1 left), putting it in your hand and returning to your Action phase
Play the Villa (+$1, $2 total)
Play the Low Road
Buy a Low Road ($1, $1 left), using the Route token to put it in your hand
Buy a Villa ($1, $0 left), putting it in your hand and returning to your Action phase
Play the Villa (+$1, $1 total)
Play the Low Road
At this point both Villa and Low Road are free, so you can repeat the process as many times as there are both of them (and if there are fewer Low Roads, you can keep buying Villas and return to the Action phase). Each time you get both every card costs $1 less and you get $1 more, and you end up reducing the total cost by the number of Low Roads in the pile plus one, and end up with $1 for every Villa in the pile after the second. You also get 2 extra Buys for the last copy each of Villa and Low Road you play. With 8 Low Roads and 8 Villas in the Supply, you could buy 3 Colonies. With just 5 Low Roads and 8 Villas in the Supply you could buy 3 Provinces.
Of course, having $10 is not easy. But if you were able to play a Villa and 2 Low Roads, you would only need $4 more ($5 total) to pile them:
Buy a Low Road ($2, $3 left), using the Route token to put it in your hand
Buy a Villa ($2, $1 left), putting it in your hand and returning to your Action phase
Play the Villa (+$1, $2 total)
Play the Low Road
Buy a Low Road ($1, $1 left), using the Route token to put it in your hand
Buy a Villa ($1, $0 left), putting it in your hand and returning to your Action phase
Play the Villa (+$1, $3 total)
Play the Low Road
Now the cost of all cards are reduced by the Low Road pile +2, and you get +$1 for every Villa after the first. Just 4 Low Roads and 7 Villas in the Supply (or 5 Low Roads and 5 Villas) would get you 3 Provinces, While 7 Low Roads and 7 Villas would get you 3 Colonies.
Given the latter possibility, it might be worth tinkering a bit. I also think a similar combo might be possible with Foreign Currency, Villa, and another discounter (Highway, Inventor, Bridge, etc.), although I haven't quite worked it out.