My 2 cents worth...
1. Don't change a thing, as many others have said "if it ain't broke, don't fix it"
2. If you do decide to change it in some way, separating it into something like PLC issues, Drive Issues, HMI issues, etc would be fine, but don't dilute it too much please. Often the answer on one system can point you in the right direction on another system.
I will often read questions/answers on systems that we do not use here on site in the hope that it will help me out on systems we do use, and often I'll learn something new every time.
3. The search functions work perfectly. It would seem that most people don't bother to use it, but that just indicates how lazy they are, not how the search actually works.
4. A mobile app would be useful about 3% of the time. If it contains short messages (i.e. Twitter), or pictures & graphics (i.e. Instagram), mobile apps are great. For text based informational systems (such as this forum), you'll find you spend more time adjusting the text size than you will actually learning anything.
For many years I have ran my own forums and been admin on many others (not PLC related) elsewhere, so I know exactly how much work would be involved in a redesign.
For me, a redesign must add something of value. If it is a redesign just to make the forum more 'modern' or 'trendy', then you're not adding anything of value.
If you're redesigning for a mobile app, then you have to ask yourself how far you're willing to go with it, and how useful you think your target audience will find it.
Everyone that uses this site is interested in the technical content, the fact it's pretty, or has the most modern looking interface on the planet won't change the content.
Just because you can, doesn't mean you should.
p.s. I would like to thank Phil and the mods for keeping us all in line everyday in a professional manner, that still allows us to express ourselves freely. You guys do a terrific job