It would be nice to have a few of the Ossassin feats backed in as class features, as they need to take a lot of feats to make their shrouds usable.
What's an Ossassin?
Also, stacking shrouds may not be such a problem later on in an assassin's life, but one thing that remains throughout the class's lifetime is targets dying before shrouds are ticked off.
If it takes two rounds for an assassin to get all four shrouds on a target, that's neat, but what then happens when another character kills the target? Suddenly that's two rounds gone and no striker damage done. And it takes another two rounds to build up the shrouds again.
It's not always the best option to let the assassin do his thing. A good example is a group I was in a few weeks ago where a controller that was built as a blaster (doing big damage to multiple targets) kept complaining that my defender was always in his targeting area. I said to him flat-out, "You have a choice, either I do my job as a defender and keep things from hitting you, or you get to fire off your powers whenever you want," he chose for me to get out of the way and he died two rounds later.
My point being is that the group can't always accommodate the needs of one character. The assassin is heavily dependent on teamwork and has no inner mechanic to deal with a lack of it. Hence why Inexorable Shroud should be an integral part of the base class. Suddenly it allows the assassin to do their striker damage without missing several turns.
The same goes for upping their damage. It might sound like 1d8+5 per shroud is a lot, but it's really not. It's only a lot when alpha striking or pulling a nova, which are usually once a day or less scenarios. If an assassin has taken all the feats and skills necessary to alpha strike a target and one-shot them, I'm ok with that as it's a serious investment and requires specific roleplaying situations to pull off.