obviously, schnorr signatures are on deck. that'll allow for cross-input aggregation to make coinjoins indistinguishable from regular transactions. that's a pretty massive development given that exchanges are beginning to target coinjoin users. estimating based on segwit's activation timeline, that could happen by early 2021 or maybe even the end of this year, optimistically.
but "useless"? that's quite a strong word.
confidential transactions (CT) to obfuscate transaction amounts seems like an attractive next step. but my understanding is it requires extension blocks or a hard fork. so.....probably not gonna be implemented at the consensus layer. there's always sidechains though. liquid (blockstream's sidechain) supports CT for example.
but "useless"? that's quite a strong word.
confidential transactions (CT) to obfuscate transaction amounts seems like an attractive next step. but my understanding is it requires extension blocks or a hard fork. so.....probably not gonna be implemented at the consensus layer. there's always sidechains though. liquid (blockstream's sidechain) supports CT for example.
These things have to run at layer 0 to get any traction imo.
taproot/schnorr will run at layer 0. CT could in theory too but there are strong reasons it won't (bloat and lack of support for consensus change).
We should have had better fungibility since day 1. Things should be mixed by default, what should be optional is making a clear A to B transaction. If we are going to have privacy, we want it to be as close to default state as possible.
taproot offers the beginnings of that. amounts and output linkability are still unaddressed at this time, but basically everything under the hood of a transaction can be hidden. cross-input aggregation (once implemented) will further provide strong fee incentives to drive users towards schnorr-based coinjoin and/or adaptor signature-based mixing transactions. wallets could offer these as automatic/default mechanisms. if most of the network is using taproot, these are pretty huge privacy gains for everyone.
unfortunately, we can't approach this issue as if it were day 1. as gmaxwell pointed out, there is uncertainty around being able to deploy even mundane consensus changes---let alone ones that are actually contentious.
What will be interesting to see is how exchanges and businesses react to this, as well as governments. The only reason governments are allowing Bitcoin to stay legal, or even neutral, is due the fact that they think they have the means to control it with efforts such as chainanalysis. Once/if BTC reached a point of actual fungibility in which the costs of trying something like chainanalysis are bigger than simply outlawing it, that is what I would predict would happen (that governments outlaw it and go into a full front attack), which will only make other governments become tax havens for BTC holders. Ultimately the price would most likely be pushed upwards but there would be an awkward period of, once again, "Bitcoin is dead" all over mainstream media.