tl;dr: bonds which have dynamic liquidity and payout.
The idea for this post was inspired by a comment in the Polkadot whitepaper (PDF). As you will hopefully be aware of by now is that the upcoming Polkadot network will be Proof of Stake and consequently earning a block reward will require staking coins (“dots” in this case).
Image source: https://polkadot.network
In the whitepaper the value of the minimum holding period and the payout are static single values. My suggestion is to make both dynamic and to allow the values to be set by the market. The suggestion of a ‘vector bond’ is that you have a single address making multiple nominations to a validator at different levels of lock-in / return.
The word “vector” should probably be replaced by “array” but the former was the first that popped into my head, and sounds more pleasant on the ears, but the word “array” is more fitting from a technical accuracy point of view.