a) Eventually (most likely next year or later), if you do not have IPv6 connectivity, there will be new content and sites added to the global internet that you will have no way of accessing at all from your v4 connected computer.
a2) Alternatively, in a year or so, you get service from a brand new ISP that has no v4 connectivity (so, you are v6 native), and you cannot access anything on the v4 Internet around the world (e.g. most things today).
b) Don't store IP addresses as 32bit integers, or as 15 character strings. Be aware that geolocation services probably don't do well with v6 addresses. Make sure your deployment platform has the infrastructure in place to serve both v4 and v6 customers (or at least v6 after a year or two) - AAAA dns records, etc.
Why would a new ISP have no v4 connectivity? They'd be connected to someone, and they would likely have v4 connectivity. Wouldn't they be able to have a private network connection to them (10.x.x.x)?
You can't eBGP route private addresses.
(well, you can, but if you do, you should be shot).
The new ISP would need it's own space to announce (it's not much of an ISP if it's got no customers to give addresses too, so it needs addresses. It's also not much of an ISP if it's only got one upstream, so it'll need PI (provider independent) space, allocated to it from it's RIR (ARIN, RIPE, etc). And that'll all be gone soon :)
a2) Alternatively, in a year or so, you get service from a brand new ISP that has no v4 connectivity (so, you are v6 native), and you cannot access anything on the v4 Internet around the world (e.g. most things today).
b) Don't store IP addresses as 32bit integers, or as 15 character strings. Be aware that geolocation services probably don't do well with v6 addresses. Make sure your deployment platform has the infrastructure in place to serve both v4 and v6 customers (or at least v6 after a year or two) - AAAA dns records, etc.