In the case of a Cleric I would just play it as Cleric == Thaumaturge. I have considered the same thing but I am not sure what you can do for upgrades beyond stats for the non-magical types - what would a level 25 fighter look like?
This would certainly help for big convention games as then you can create warband from 2-3 players with one "big guy" per player. I am thinking of the same thing for my 12 player game at Cangames.
The issue there would be more in how the spells and abilities would be geared for.
For example, things like healing spells and things would work well for one of the domain archetypes, each would have its own thematic focus. The reason for having bits from the others beyond the main one is that it would work well to let someone develop the cleric into a better fit for whatever god or purpose they want him to go after.
For a top level fighter, you could have different versions of them that fit different archetypes. From the samurai who could probably pull the move and strike in a single heartbeat. Think of something like being able to mark a character attacked and essentially pull a second free attack on that target at the end of the turn to represent the people falling to pieces. Another could be someone like the viking from Stamford Bridge or Benkei from Japan with his standing death, or perhaps going for a more commander type giving buffs to his men...