Plausible Roll-Defense Mechanic?

What you are approaching in either case is something similar to White Wolf's World of Darkness Role Playing Game combat mechanic. This is, unfortunately, not a good thing. One of the best things White Wolf did with their later title Exalted was to make defenses un-rolled to speed up gameplay. What they did was simply take the average number of successes that a character would get on a Defense roll and assume that they always roll that number.

For example, if a character has 10 dice and a 50/50 chance of getting a success on a given die then they would have a defense value of 5. Attackers still roll dice as normal, but defenders are treated as having a static value which must be overcome. This static value is frequently modified by situations/weapons/abilities, but it serves as a starting point.

You could do something even simpler and cause defense values to simply subtract successes from the attack. Any successes remaining would help to determine the strength of the resulting hit in some way. This approach gives you an easy way to assign defense values to things: "How often do I want a given attack to hit this character?" The math afterwards is quite simple.

Now I know that this is getting away from rolled defenses as-such, but I thought it might be instructive to think about games that have walked this path before.

One of my games I'm working on have an attack/defense mechanic that I really like. Without getting into specifics, because I plan on expanding it to a full system. I find that attack vs defense then decide damage based on how successful the attack was works very well.