Levels - How about two-dimensional advancement?

[Umbran]

If horizontal advancement is just that, a widening of options without increasing effectiveness, there won't be similar problems.

Thing is, that's not possible. In any given system, there will always be optimal choices, synergies and combinations that are more effective than others. When you add to the breadth options, that means characters are able to use more of those synergies. It is a fact of systems, in general.