Reverse proxies usually present internal applications to the internet. In these cases you will usually want the certificates to be trusted by a wide range of public computers - which necessitates certificates issued by public CAs such as Verisign etc. These CAs absolutely will not give you a delegated subordinate CA for this purpose.
Wildcard certificates provide similar functionality but are high value targets to attackers - discrete certificates with limited subjects and applications are recommended here. Lets Encrypt can help you do this in a scalable and cost effective way.
If you do not need public trust, I would recommend the same practice but issued from your internal PKI. There are a range of considerations here including making sure your internal PKI's CDP and AIA URLs are available to all validating clients, possibly also through your reverse proxy solution.
I would run a country mile away from issuing CA certificates to reverse or forward proxies in my opinion.