If I was given the job to redesign the US coin system (ignoring all the extraneous expenses and public confusion attached), I'd go with something like this:
Value | Nick | Diameter | Thickness | Edge | Usage | Obverse | Reverse | Color |
.01 | Penny | 18mm | 1.4mm | plain | limited | Lincoln | Union shield | copper |
.05 | Nickel | 20mm | 1.5mm | reeded | wide | Jefferson | Cape Disappointment Lighthouse | dull silver |
.10 | Dime | 22mm | 1.6mm | plain | wide | FDR | Marine Corps War (Iwo Jima) Memorial | silver |
.25 | Quarter | 24mm | 1.7mm | reeded | wide | Washington | continue 'America the Beautiful' theme | dull silver |
.50 | Half-dollar | 26mm | 1.8mm | plain | wide | JFK | Apollo 11 LM on lunar surface | silver |
1.00 | Gold dollar | 28mm | 1.9mm | 10-sided | wide | decd US presidents/prominent legislators & governors | American eagle motif or state seal | gold |
2.50 | Two-fifty | 30mm | 2.0mm | 25-sided | limited | decd US civil & equal rights/immigration figures (+unintended) | relevant to each face | inr milled; gold (inner 80%) / silver (outer 20%) |
5.00 | Five | 32mm | 2.1mm | serrated | wide | Series 1: decd US science/exploration figures; Series 2: decd prominent US captains of industry | relevant to each face | silver |
*$1.00 bill still in wide circulation until the coin catches on; $2.00 bill removed from circulation; $5.00 bill remains in circulation
If things are a buck-99, with taxes the total's going to push over two bucks. So a $2 coin doesn't make a whole lot of sense. I suppose the same could be said for a dollar, though. I just like $2.50 better. Couldn't bring myself to make a tree-fiddy, though.
e: (yeah, I put way too much thought into this)