Replace "unplaced units" with currency

[stevegrossi]

i.e. instead of gaining 1 unit per 3 tiles held each turn, you gain 1 silver per tile held, and new units cost 3 silver. This provides some interesting mechanics:

• It helps losing players: if you only hold 2 tiles, you can still purchase a bonus unit after 2 turns
• It provides a way to slow down winning players, keeping the game more interesting. I'd like to make units purchased in the same turn increasingly expensive (e.g. the first is 3 silver, the next 4, then 5, 7, 10, 15, and so on) so the first player to control the largest number of regions can't just immediately cash their bonuses in for an easy win.
• It makes it possible to spend currency on things besides units. Some ideas
  • Cards that provide some kind of bonus (like Risk)
  • Tile improvements: either one-per-tile or one-per-player on a tile they own
    • Stables might grant an additional move per turn
    • Mines might grant extra currency per turn
    • Castles grant a defensive bonus on that tile
    • Harbors might let you move from any coastal province with a harbor to any other
  • As another way to avoid a tipping point where the first player to gain an edge in units always wins, I'm interested in a concept of upkeep. For example, every unit you own costs 1 silver per turn. This might force players to grow more slowly, build improvements to support larger armies, etc.

I am wary of turning Sengoku into Civilization or another complicated empire-management game. I want every feature added to improve the game in some way, not just make it more complex, since I want to keep Sengoku as easy to learn as possible. So I'll approach these ideas hesitantly.