I'm a bit leery of any UI overhauls done to implement greater mobile support. I think it's probably best to implement a stylesheet that will do special things like relocate the profile sidebar when viewed from mobile and change the page width instead of messing with the existing page layouts. I've seen a lot of modern mobile-friendly forum UI overhauls and I have to ask that you please do not do inflict drastic changes on the normal forum UI like that. As far as table display on mobile is concerned, I think we will largely have to just accept the tables be, since most mobile users would probably sooner prefer to have to zoom in than deal with broken tables. If we are still inflicting page resolution limits for pages with tables, we should probably implement horizontal scrollbars for tables while also adding a special option to expand the table back to full size (raising the page resolution), since it can be a real nuisance to have to mentally piece together the entire table since you're only seeing small pieces of it at a time.
As for Discord integration, please don't. Aside from real concerns over user-tracking behavior, Discord has a tendency to consume a lot of CPU and RAM (especially when run through a browser) as well as unnecessary bandwidth (This is rather relevant if you are using a cellphone's internet connection.) and I can confirm that having discord active makes old computers run slow as molasses. This will essentially ruin the usability of the website for anyone on a slow PC (you know, the ones with slow CPUs and only 4GB of RAM) so I strongly recommend against Discord integration. Make a new, snazzy discord banner if you prefer, but right now this website actually runs very well on older and slower computers, so let's not ruin that by bundling discord into the webpages.
As for implementing a URL pop-up for the link and mailto buttons, I certainly support that and my recommendation would be to actually just use the easiest and laziest solution, which is the good old javascript prompt() function (like prompt("Please insert link URL:")). Pretty much every browser supports it and it's certain to appear properly and work right regardless of your browser's particular UI needs, without futzing with divs and css and resolution-specific concerns. Really, it does the job and there is no need to overcomplicate it.
As for snazzing up the website header area, I have no strong opinions on this and go right ahead if you prefer, but I do recommend not enlarging the overall header area, since that just consumes valuable screen space that would be better spent on letting people see more of the page proper. I also think it's best to make the new header not something too demanding for attention since that will just become a minor nuisance and an unwelcome distraction from reading the page contents. Other than that, my major recommendation is actually to try to reduce any empty vertical space in the header area so that there is more screen space for the more useful page contents below.
On the subject of adding more table code functionality, I'm all for it so long as it does not become a breaking change, but if need be we can probably just make new table tags. Really, I'd only consider it a problem if it either breaks existing tables or makes existing habits of creating tables suddenly not perform as expected.