If you want this to happen, which it seems you do, Let's Encrypt is probably the wrong place to be arguing for it at this point, you'd need to talk to the trust stores, as they'd have to substantially weaken their oversight over the Web PKI in order to make something like this practical.
The major trust stores (where you'd want to start) are Microsoft, Apple, Google and Mozilla. If you can get those four on board you might well get somewhere with this. They're responsible for the trust of public CAs like Let's Encrypt by most popular web browsers and, (indirectly in Mozilla's case) operating systems.
To give you an idea of why the root trust stores don't want somebody they have no relationship with issuing whatever they want under an intermediate as you're envisioning, consider what happens when inevitably, but perhaps purely by mistake, you issue a certificate which contains a technical error.
Perhaps it has a stray NUL character in a text field, or it contains a nonsensical policy OID. Unfortunately, and unknown to you, this error causes a popular program to misbehave, maybe it's a web browser, or a mail client.
Today, under current rules, the trust stores can demand that the ultimate issuing CA revoke the offending certificate and ensure the problem is prevented from happening again. If it's unresponsive or its attempt to prevent future problems is unsatisfying, they can just revoke trust in that entire CA. If there is a cross signature they can insist the cross signature is revoked in order to give effect to the loss of trust. This really happens, albeit usually not for minor technical errors since it would make sense to just ensure the error stops happening.
But under your proposal this mis-issuance would have taken place at some unknown location, perhaps on some Raspberry Pi in a closet, the person who issued the broken certificate can't be contacted, or may not think fixing this is important right now. So Let's Encrypt (in your example) are ordered to distrust this intermediate. That fixes the problem, once, but it's not permanent. "Huh, why is my issuing CA broken? I guess I will make another one" and the cycle continues.
So, you will need to show the trust stores how your proposed new regime could achieve their goals. It is possible you can do that, but that's probably where you need to start, not at Let's Encrypt. And frankly, you may struggle to even get Apple and Microsoft to notice you exist.