Run without a derailleur hanger? I'm building a bike on an old aluminium frame that is missing the rear derailleur hanger. I'm putting an IGH on it, so I don't need to use the hanger, but the dropout seems too thin without it. Can I do without, or is it necessary to avoid bending the dropout somehow?
Most aluminum frames have a replaceable derailleur hanger, which typically is part of the dropout itself as you can kind of see in this image:



Without it, the dropout is indeed too thin and your spacing is messed up, not to mention that the strength of the dropout is decreased. You need to identify the correct replacement hanger and get it fixed.

Also, I don't know what IGH you're using, but it will most likely require some sort of chain tensioner as well, and without sliding dropouts or an Eccentric Bottom Bracket, you're probably going to need a spring-based chain tensioner that will bolt on to the derailleur hanger: