There are none. Playtesting has shown that characters still hit quite easily at Epic even without expertise, for example.
This has nothing to do with expertise being a feat tax. This goes to if expertise feats are needed or not.
They are simply build choices. 'Feat Tax' is an ignorant term made up by those who want to show the designers made a mistake and are trying to fix it because they were smarter than the designers by finding the problem.
This also has nothing to do with expertise being a feat tax.
In the end, feats can help mitigate poor party composition or poor tactics in order to help a group eek out a 'win' but they're just choices.
The feat system works because all feats are approximately the same power level. This means that feats are build and class dependent. Some feats will be more or less useful based on those choices about your character. For example 'Heavy Blade Opportunity' is pretty useless for a Rogue who wants to use light blades.
The problem with 'Expertise' is that at level 1-14 it is just as you claim, a "build option", but by level 15 becomes more powerful than any other feat out there you can pick in terms of offense. Because it is so much better than any other choices you are foolish to NOT take it. This is what makes it a feat tax. I challenge you to show any build on the charop boards that does not include expertise by level 15 for +10% to hit. When a feat becomes this universal there is something wrong. And no I'm not arguing if we should have expertise or not. The plain fact is we DO have it and what do we do about it. For me, I've removed it and I'm adjusting all monster defenses down to account for it. This makes ALL powers better, not just those that work with a weapon or implement....Dragonborn breath comes to mind immediately or Bullrush just to name two.
This also explains why almost every caster wants a "weaplement". Weapon Focus is +1/tier to damage and doesn't have any stat requirements. The other choice for casters is to take those feats that give you +1 damage to 2 different damage types (such as thunder and lightning). The problem with these is they won't apply to every power you take (unlike weapon focus for weapon users) and they have stat requirements which may give you MAD issues depending on your class and build. This is another case of a "strictly superior" feat being used where possible. What they should have done is just had "Implement focus" just like weapons and you can pick it with your...well...implement such as totem, holy symbol, etc.