In 3rd edition, we kind of had that option. Back then, the Prince was an upgrade option off the Lord template. There were tons of options. However, instead of giving the Prince a +4 inv, the MoT conferred an additional wound.
bliss-giver, how many points do you think is fair for your desired model? (i.e., prince with both a 4+ inv & T6).
Our Princes are the last of these model types. A "7" is just incomprehensibly good. Only Abby, Avatars, and a few other legendary models can compete with him. In addition, he also has enhanced stats across the board when compared to other marine HQs, so I really don't see him as having merely "acceptable" stats. Then, to top it all off, he's seriously underpriced.
bliss-giver, I think what you're wanting is more of a god-model. Actually, we had these in 3rd edition with our Greater Daemons. They weren't cheap, and for this reason, nobody took them. Well, some people did. They actually weren't too terrible. But back on topic: Let's say you wish to create a demi-god Prince. Don't you think the cost will likely be prohibitive for fielding him with an competitive notion in mind? The fact that our prince is slightly weaker in defense when compared to his offense is the one saving grace our opponents have. Honestly, I can't see giving our Princes anything else without raising the price to such a lofty number that he's no longer an effective option. Already, he's generally regarded as one of the best HQs in the game.