Heroku got bought by Salesforce several years ago.
If you have 12-factor apps then you have a fighting chance of moving off it anyway.
Apart from Dokku, I'd say Cloud Foundry is the closest next environment that you can install and operate directly, though it's an 800-pound gorilla by design. But there are fully hosted services for it (eg. Pivotal Web Services, IBM BlueMix, SwissCom Application Cloud). There're also semi-hosted options (Rackspace Managed Cloud Foundry) and IaaS-provided installer kits for AWS, Azure and I think GCP as well. You can also buy commercial distributions from Pivotal, IBM, Atos, SUSE, IBM and SAP.
Disclosure: I work for Pivotal, we sell Cloud Foundry and Kubernetes distributions (PAS and PKS).
If you have 12-factor apps then you have a fighting chance of moving off it anyway.
Apart from Dokku, I'd say Cloud Foundry is the closest next environment that you can install and operate directly, though it's an 800-pound gorilla by design. But there are fully hosted services for it (eg. Pivotal Web Services, IBM BlueMix, SwissCom Application Cloud). There're also semi-hosted options (Rackspace Managed Cloud Foundry) and IaaS-provided installer kits for AWS, Azure and I think GCP as well. You can also buy commercial distributions from Pivotal, IBM, Atos, SUSE, IBM and SAP.
Disclosure: I work for Pivotal, we sell Cloud Foundry and Kubernetes distributions (PAS and PKS).