Organize your Built-on Applications such that the Ruleset stack for any Client Application positions all Pega-provided Rulesets (including those from Pega's applications) at the bottom of the Ruleset stack, and Client Rulesets above them. Pega Applications are not tested in scenarios that inject other non-Pega Rulesets in between Platform and one of its Applications.
With this approach, any Class introduced in a Client Ruleset can direct-inherit from a Class provided by Pega Platform or a Pega Application. New or improved capabilities added to those Pega Classes via upgrades become available to your Client-layer subclasses through Inheritance.
Similarly, consider defining distinct Built-on Applications for Client Enterprise and Reuse (formerly, "Framework") layers so that their Rulesets are beneath those from your Case layer. Classes in your Case layer can follow direct- (or pattern-) inheritance paths through more generic Client Rulesets, and then direct-inheritance paths to Classes from Pega Rulesets.