There are a number of good ideas floating about here, but there are a few issues with what's being suggested.
Firstly, there seems to be an assumption that centralisation is a "good thing" (and therefore an implied assumption that decentralisation is a "bad thing"). This is a hangover of a lot of history books until about the 60s (and beyond in some places) which saw the western centralised states as the obviously supreme method of rule. Truth be told, centralisation wasn't always a good thing, and in the middle ages it wasn't even feasible in anything other than the smallest kingdoms. With kingdoms that could take weeks or months to cross (or may have areas inaccessible for parts of the year) some local delegation was a necessity. In these situations having powerful and autonomous nobles could be essential, not least for purposes of defense. Another point to think about is that it was much more impressive to have a number of powerful dukes swearing fealty to you than a group of weak counts.
The problem with the king taking greater rights over his vassals is that in many cases this could undermine his own rights. If, for example, the king declares he can remove nobles at will then this also undermines his assumed right to rule without question. On the other hand, if the king upholds his vassals rights to their titles, then his own legitimacy is strengthened. By "centralising" his realm a king could easily undermine his own power.
It should also be noted that royal-vassal authority was a two way street: while a king did claim authority by visiting his nobles, this also endowed the noble with authority through the heightening of his connection with the king. This was one of the reasons why nobles would travel to they court of their lord: they wanted to be associated with the king, the main source of authority in the kingdom.
Nattai mentions itinerant kingship, which was the dominant form of rule for most of Western Europe in this period, not just for kings but for dukes, counts, bishops and even the pope. I'd love to see this implemented as a king moving around his personal demense and other parts of the kingdom. It would quickly get complicated however as almost all noble courts did the same thing, travelling around their own territory and to that of their lords. Even a lowly count with a one province demense would travel to his lord's court occasionally.
While centralisation should be possible after a fashion, through installing and agrandising the public positions (those of royal officials) there should be a lot of problems associated with an attempt to do this.