Just about every state and nation we live in carries debt of some sort.
How about the empires we build in this game ?
I see a few options for Debt :
1 . Monopoly mortgage model. Econs can be mortgaged for a small amount (maybe $15)
Industry (maybe $35) and Science (maybe $75) . Mortgaged infrastructure items require no interest, but produce nothing until the mortgage is paid, or the star is captured by another player.
SimCity Model. Infrastructure keeps producing , but you got to pay interest (rating agency considers amount borrowable, war activity status) for the floating interest rates you get.
Real estate model, borrow up to 80% of the value mortgaged, nonpayment subjects your infrastructure to forclosure
Debt can make this game more interesting for the veterans.
The complexities are likely to discourage the newbies, who already have a steep learning curve.