Nice list, thanks for sharing! Shows there’s huge potential of this feature and a huge amount of work if we want to unleash all of that. 
Update
There’s a couple of things that need to be done now to be ready for a mainnet release. The below ones are the main open areas where we will spend most of the engineering time. We can expect 2-3 months until we can release, but this is a rough indication, please do not use this as a firm release date.
Assurance
One of the big items here is to increase our assurance of correctness of the implementation. We have already done one external round of code reviews with a world-leading code audit company. We are planning to do another round when the changes that we still want to implement are complete. This will be again with a top-notch external review firm. Review reports will most likely be published. And of course we will be doing lots of testing on our own. For a feature like threshold ECDSA, which will protect large amounts of value, such efforts for quality assurance are crucial to ensure that the chance of security issues and thus the risk of value being lost is reduced to an acceptably tiny value.
Performance
In addition to these assurance-related efforts, we still need to work on improving the performance of the implementation. We are currently running the system with the test key on a 13-node subnet. For the production key, we decided to start with a 34-node subnet for the security / decentralization we want to have. This has a major implication on performance, i.e., it will be quite a bit slower there. In order to reach, what we think is an acceptable level of performance, we need to put some more thinking and engineering effort into getting good-enough performance on such larger subnet.
We will keep you posted on the progress!
B.t.w., user @flyq from our community has (against all warnings) already taken the risk to perform a Bitcoin mainnet transaction with the threshold ECDSA test key. After manually crafting a bitcoin transaction to transfer the funds his canister has received back out of the IC again, no Bitcoin was lost and this was the first successful threshold ECDSA-based Bitcoin Mainnet transaction roundtrip on the IC ever! Congratulations! 
The warning still applies to not attach any value to the current t-ECDSA test key. Any experiments are at your own risk if you choose to ignore this warning.