There are still modules which use 1.6 or 1.7 and do not need that. The idea is to run jdeps on each artifact of a modul, add the relevant java.* modules to the dependencies of the module and upgrade to version 1.8. In a first only none-jdk modules and none which depend on system dependency.

Full list:
./system/layers/base/sun/scripting/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="sun.scripting">
./system/layers/base/sun/jdk/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="sun.jdk">
./system/layers/base/ibm/jdk/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="ibm.jdk">
./system/layers/base/io/undertow/core/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="io.undertow.core">
./system/layers/base/org/apache/log4j/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.apache.log4j">
./system/layers/base/org/apache/commons/logging/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.apache.commons.logging">
./system/layers/base/org/fusesource/jansi/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.fusesource.jansi">
./system/layers/base/org/slf4j/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.slf4j">
./system/layers/base/org/slf4j/impl/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.slf4j.impl">
./system/layers/base/org/jboss/stdio/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.jboss.stdio">
./system/layers/base/org/jboss/jandex/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.jboss.jandex">
./system/layers/base/org/jboss/vfs/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.jboss.vfs">
./system/layers/base/org/jboss/staxmapper/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.jboss.staxmapper">
./system/layers/base/org/jboss/as/domain-http-error-context/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.jboss.as.domain-http-error-context">
./system/layers/base/org/jboss/as/core-security-api/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.jboss.as.core-security-api">
./system/layers/base/org/jboss/as/version/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.jboss.as.version">
./system/layers/base/org/jboss/as/domain-add-user/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.jboss.as.domain-add-user">
./system/layers/base/org/jboss/as/management-client-content/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.jboss.as.management-client-content">
./system/layers/base/org/jboss/as/deployment-repository/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.jboss.as.deployment-repository">
./system/layers/base/org/jboss/as/threads/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.jboss.as.threads">
./system/layers/base/org/jboss/as/patching/cli/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.jboss.as.patching.cli">
./system/layers/base/org/jboss/as/controller-client/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.jboss.as.controller-client">
./system/layers/base/org/jboss/as/standalone/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.jboss.as.standalone">
./system/layers/base/org/jboss/logging/jul-to-slf4j-stub/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.jboss.logging.jul-to-slf4j-stub">
./system/layers/base/org/jboss/logging/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.jboss.logging">
./system/layers/base/org/jboss/invocation/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.jboss.invocation">
./system/layers/base/org/jboss/dmr/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.jboss.dmr">
./system/layers/base/org/wildfly/embedded/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.wildfly.embedded">
./system/layers/base/org/wildfly/extension/core-management-client/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.wildfly.extension.core-management-client">
./system/layers/base/org/wildfly/security/manager/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.wildfly.security.manager">
./system/layers/base/org/wildfly/security/elytron/main/module.xml:<module xmlns="urn:jboss:module:1.6" name="org.wildfly.security.elytron">
./system/layers/base/org/jboss/logging/commons/logging/main/module.xml:<module xmlns="urn:jboss:module:1.7" name="org.jboss.logging.commons.logging">