Governments world over will be looking for ways to tax cryptocurrencies because they are missing out on huge tax revenues, I'm skeptical as to how crypto to crypto would work. I would think the best way to tax would be similar to stocks where profits are only taxed when stocks are liquidated. I guess however that may not work for cryptos as they can be used as a means of payment.
Governments already have a way to tax crypto, using the same mechanisms as stocks. The common understanding that stock profits are only calculated when liquidated appears to be misguided. Apparently if you trade one stock for another (or one asset for another asset) that is also considered a taxable event and taxes due, with the amount based on market value at time of transaction. Look at it this way, governments have had many decades to see and close loopholes with stock and asset trades, they just apply the same rules to bitcoin and no new regulation is required, just some statement that bitcoin is classified as an asset, which they have done across major jurisdictions.