Driving from work today I started thinking about RoE and I remembered something which was completely normal for most medieval rulers (as it is for some modern superpowers) and that's debt.
Examples are too numerous to count, but since Roman times up until the present (Immediate present consideringBush's $700 billion "Save the big bankers" plan) emperors, kings, counts, president etc funded their armies, housing, infrastrucuture and courts with "I owe yous." Additionally, it was quite common that such debts were never paid (anyone seriously thinks that the US will ever be able to pay off its 10 trillion dollar debt?) or that "devalued" money was used (lesser quantities of precious metals in a coin than usual) which lead to inflation, bankrupcies of creditors, market collapses (Rome had a 300 year trade deficit with Persia for instance) and all kinds of nasty economic and social problems.
Birthright, however, seems to be operating on a total debt exclusion economy. You only spend what's in your treasury which, although practical, isn't really that realistic. An army won't desert (immediately) after not being paid, courts will still function, merchants will deliver food to the castle hoping for future payments and so on.
No idea how exactly this could be implemented but it would be fun.. Imagine a king after a long campaign facing a riot and being forced to sell some assets (law holdings, titles, counties) to his creditors. Many a merchant bought himself the way to nobility that way
