In my opinion, introduction of innovations through cloning is the number one threat to bitcoin.
I think there will be a clone that includes at least the following features:
a) mining for regular coins
b) mining for bond coins (i.e. tradeable securities that the blockchain converts into regular coins at a future date)
c) splitting of bond coins into derivative coins (i.e. tradeable securities that the blockchain converts into regular coins at a future date if certain block chain conditions are met (e.g. difficulty levels, transaction volumes). A set of derivative coins which spans the entire domain of future conditions is equivalent to a bond coin.)
d) Threshold-protected wallets (wallets that allow for daily send limits and an unlimited emergency cash-out address. Both the send limits and the cash-out address would be specified by the user at wallet creation and could not be changed subsequently.)
e) other features people haven't thought of yet.
It would be much better for digital currency if these features were developed as a fork in the bitcoin blockchain rather than as bitcoin clones. Development of successful clones undermines faith in the ability of digital currencies to store of value. Development of a successful forks allows pre-fork currency holders to have their wealth grandfathered in. Forking preserves the ability of digital currencies to store value, while allowing for innovation.
What is the point of bondcoins, and how would you enforce the threshold.
Short Answer:
Bond coins facilitate financial contracts which span a long time period.
For example, in this thread [http://forum.bitcoin.org/index.php?topic=18205.0] a company is issuing private bonds to fund a project. The company promised to pay 1 bitcoin on Dec 31, 2011 for every 0.8 bitcoins invested by bondholders today. Currently, the company needs to accumulate current bitcoins to pay off this debt. Suppose that the company accumulates bondcoins with a maturity of Dec 31 instead. Based on the poll I made here [http://forum.bitcoin.org/index.php?topic=25705.0], bondcoins with a half a year maturity might sell at a discount of 10% (i.e. 0.9 bitcoins trades for 1 half-year bondcoin). If so (and assuming the company has a constant revenue stream), the company would save around 5 percent of its financing costs by accumulating bond coins rather than bitcoins. This would reduce the interest rate the company faces from 25% to 19%.
This is just one example. Essentially, the same logic applies to any long-term contract demoninated in bitcoin.
Longer Answer:
A second point of bondcoins is facilitating the creation of derivative coins. Derivative coins address issues with compensating large merchants for adopting bitcoin and helping merchants manage bitcoin exchange rate risk. My ideas about derivative coins (aka contingent claims) are described here:
http://forum.bitcoin.org/index.php?topic=19130.0Regarding the threshold, that is a programming question. I'm not a programmer. I bet the current developers could manage it if they set their minds to it, however.