Interesting market effects
I can picture all sorts of interesting market effects happening to IPv4 addresses.
A pricing difference between "clean" and "dirty" addresses. Addresses that are on blocklists for spam or botnets should trade at a lower value than ones that are clean. The question is, how can a buyer assess the "quality" of an address? Without transparency that market effect can't work. Are there already consultancies or services that can help you buy a "clean" block of IPv4 addresses?
Some companies might be more relaxed about buying dirty addresses, depending on their use case. Would it matter if Netflix would buy a block of addresses for its streaming servers that happens to feature on spam blocklists? I would think not. Meanwhile, if you're planning to set up a new email service provider you'd want to buy the cleanest blocks available.
On overall view, not tomorrow but perhaps next decade, that due to the lack of regulation, legacy ownership issues, legacy transparency issues, rampant abuse etc. that IPv4 traffic is dodgier than IPv6 traffic. It may lead to a situation in let's say 2030 that people reaching your server from IPv4 addresses will need to go through a CAPTCHA (or other Turing test) whereas standard IPv6 traffic doesn't. At the moment Google is a bit stricter about email traffic coming from servers over IPv6 than over IPv4 as blocklists for IPv4 are easier (and more established) than for IPv6. Will that stance reverse at some point?
A steady rise in value in IPv4 addresses up to a certain point until it hits a peak and then a fairly sharp drop. At the moment about 33% of all traffic is IPv6, once that reaches a certain level the value of IPv4 will suddenly decrease and it tips from a seller's market into a buyer's market. As a seller of an IPv4 block it makes sense to hold out for a bit longer, but not too long because it will suddenly drop quite rapidly as every holdout will sell their block while it still has value. Like most markets, it doesn't matter when the real tipping point is, it matters when people think we have reached that tipping point. The perception is enough to create the tipping point.
At some point the majority of home and office users will have a Dual Stack (IPv4+IPv6) or DS-Lite (IPv6+IPv4 behind NAT) connection. At that point the only people still interested in IPv4 blocks will be owners of servers who need to make sure that IPv4-only users can still reach them. This might mean a release of IPv4 blocks by ISPs who sell it to server owners (from massive cloud companies to smaller hosters). If you're an ISP that always had a good abuse department your blocks might be worth more than those of ISPs that are infamous for their lax approach to botnets, spammers, open relays etc.