That's basically the solution they already implemented for Blade Pact Warlocks but with an extra item destruction twist. As such I can't say it's a terrible idea, but there are some issues. How does it interact with sentient weapons? Does the base damage die change if you imprint a rapier or dagger? For that matter, what restrictions are there are on what weapons can be imprinted? It can't just be proficiency or else a Soulknife with a multiclass dip can be imprinting greatswords and halberds.
Still, it's in the direction I suspect they may have to go.
Well, for most of this I would say "how crazy do you want to get, Dungeon Master?"
For me, sentient items are always immensely powerful artifacts. Even attempting to destroy one of those is going to be a massive challenge and I'll have to make up rules for essentially having a duel of souls if the player wants to try it.
Changing base die? I'd allow it. The idea is to let them take the weapon and make a psychic version of it. So, Rapier would be fine, but it might lose the light property if you did.
And, I never consider multi-class. Combine that with the idea that I think a psychic greatsword of halberd sounds dope, I would probably allow it, and again, it would lose light, gain two-handed. ect.
But, this is just how I'd do it off the cuff.
Edit: And before we get a discussion about "sneak attack with Greatsword" I have a point. To multi-class they will likely give up more than one level, for fighter they usually take at least 2. Which, puts them behind on sneak attack, usually by a single d6. Meaning the advantage of the d6 they gain on the weapon is negated by the d6 they lost from sneak attack. IMO.