I was thinking about Eldritch Knight the other day and it dawned to me, why not make some Pre's have more than just point and class requirements. Why not have Pre's that require you to have been a particular class with a particular Enhancement set as well.
An example would be a Blackguard. Basically a dark paladin, who in the previous life was a paladin with tier 5 necromancer enhancement. This would, in your next life open a new enhancement tree with abilities that are catered for a multiclassed player who wants paladin and undead.
*despite the obvious that blackguards aren't anything like that in PnP; this is an example to illustrate the idea concept.
The question is now, what do you think those requirements should be and what kinds of new trees would you want?