Assassins really only have two main weak points currently (three if you care about multiclassing them, but I don't so I'll leave that to others to deal with).
1) Their damage mitigation is weak and
2) their dps is far behind bard/paladin.
Damage Mitigation:
A pure assassin can already reach dodge cap easily enough, so I don't see the point of the additional dodge you propose. Assassins are currently doing fine in the damage avoidance department. What they need help with is damage mitigation. Even with max damage avoidance, if you have low PRR, then you're going to take some heavy spike damage. Couple that spike damage with poor self healing (and no, heal scrolls are definitely not sufficient for the current endgame, so having UMD does not qualify as good self healing) and you can find yourself having a hard time recovering.
Solution: Increase the light armor mastery bonus to 5/10/15 PRR, or at the very least 3/6/10. I saw you increased it to 2/4/6, but that's still not significant at all. Thematically, this makes more sense imo. Light armor mastery makes me think you've learned to maximize the protection you get from light armor (i.e. damage mitigation, not damage avoidance). So this should be more about PRR than dodge. If you think a straight increase in PRR to the current version is OP, then take the dodge part away entirely and just make it 5/10/15 PRR.
You offer an interesting alternative to damage mitigation in the form of the melee power debuff, but I have some questions about that. Do all mobs have melee power? If they start with 0 melee power and get the debuff, do they end up with -10 melee power? Basically does this reduce mob damage whenever it is used? Also, as a mob debuff, this is effectively a buff to other classes, many of which may not need it (e.g. bard, paladin, fighter, barb) and only further shifts game balance in favor of them over an assassin. I like the idea of a debuff to mob damage instead of straight PRR on an assassin, but consideration needs to be taken about how that affects other classes and overall game balance. Some more damage mitigation alternatives would be nice (e.g. a paralyzing poison, obviously with a save that would be effective in endgame content and would continue to be effective once the level cap is raised to 30).
DPS:
If I remember correctly, you yourself, Severlin, stated that bard/pali were the new baseline, so a comparison to them is warranted at this point. Assassins measure up just fine when compared to any class that has not had a revamp yet, but they are far behind bard/pali/barb. I've played an assassin, swashbuckler, and paladin at endgame (see the links in my sig for the assassin and swashbuckler if you want specifics about the builds) and there's no question that the assassin is behind the other two (I won't even go into how my swashbuckler is much better at assassinating than my assassin, and the assassin has a 74 DC so it's not a problem with the build).
Solution: Add some melee power in the tree somewhere. Put 5 melee power in the capstone and make stacks of measure the foe fade 1 stack at a time every 6 seconds. About 3/5 of an int based assassin's dps comes from sneak attack damage. The fact that melee power already affects sneak attack damage provides a simple solution to increase their dps and still have it based primarily on sneak attack damage instead of crits like bard, paladin, and barb. Measure the foe fading 1 stack at a time would make those bonuses more consistent, and that's not a bad thing if we're trying to bring assassins up to the same level of power as bard/paladin/barb. It would give assassins a reason to be in sneak mode at least some of the time while making these bonuses last long enough to actually be relevant without having to spend too much time in sneak mode.
TWF:
I also have to add that TWF being behind every other style is most definitely part of this discussion and certainly belongs in this thread because it is the obvious fighting style for an assassin. There's no point in talking about an assassin revamp if you ignore that they use an inherently weaker fighting style that is in need of a revamp as well.
Solution: The simple and obvious solution is to add melee power just like the other styles get. Considering that swf gets not only 1.5 stat damage, but also 30% attack speed, in addition to 10 melee power, I really don't understand what the problem is with giving twf 10 melee power.
Sneak Mechanics:
The same needs to be said about sneak mechanics, no point in revamping assassin if you ignore that a core feature of the build is broken. No other class or build has a built in incentive to sneak, so a sneak mechanic revamp is definitely warranted. It works fine in a group but not solo. Here is what should happen, and did at one point in the game's history, so it really shouldn't be that hard to get back to it. When I assassinate one mob, his buddies in the local vicinity should go into search mode and start looking for me. Maybe they should even get a bonus to spot and listen. They should NOT automatically detect and agro on me, which is what currently happens. This really doesn't make any sense. It is possible to shed agro of melee mobs with appropriate tactics (lose line of sight and go into sneak mode again). But the automatic agro is especially problematic when it comes to archers because once they agro on you, there's no going back into sneak mode since every arrow breaks you out of sneak mode. This really needs to be changed. The ideal fix would be to make it so that when you drop into sneak mode, they don't automatically know where you are even if they were agroed on you when you went into sneak. Again, give them a bonus to spot/listen, but, thematically, an assassin should be able to slip into the shadows and disappear.
Capstone:
The capstone needs some adjusting.
Blackheartox's summary of the new capstones pretty much says it all. The proposed assassin capstone is pretty weak in comparison. Dex is useless for a pure assassin and they already have more than enough reflex save, so drop both of those completely.
Solution: Make it +4 int and, as mentioned previously, add 5 melee power.
Execute:
This is far too weak to be useful at endgame. My initial thought is to turn this into a second assassinate ability, effectively giving assassins two instakills, but I don't know how to do that effectively and with balance.
Nicely Done:
Great job on giving assassins the option to move at full speed while in sneak mode. This will greatly enhance practical gameplay in groups. Also, nice job on reducing the animation of assassin's trick. I never use that ability as it currently is but I will with this change.
Looking forward to a revamp of my favorite build. Thanks for the discussion Sev.