In terms of issuing more of the same asset, for now you would simply issue a second asset (perhaps with a similar name) and then combine these assets together at the user interface level, i.e. present them as one fungible quantity.
The problem with allowing additional asset units to be created in a subsequent transaction is the security - it could only work if those units were issued by the same private key. In that case you might as well just issue a much larger quantity in the first place, and keep most of the issued units at a 'cold storage' address for maximum safety.
In terms of deleting some units of an asset, you simply send them to a fake address for which nobody has the private key. That ensures they can never be spent.
To summarize - at the blockchain level each asset is a separate one-time issue. But you still have a lot of control over how you present the blockchain level information to the user, and this could include both: (a) combining separate assets together, and (b) excluding asset quanties which are sitting in cold storage or have been sent to a specific unspendable address.
