Some things I think would be nice:
-Direct messaging (to network and build professional contacts); it goes without saying, but the messages should be encrypted like in Facebook, Whatsapp, etc.
-No ad option like KootK said.
-Better mobile view; it's almost totally unusable.
-Ability to paste images directly instead of drag/drop.
-Quality of life improvements, like being able to turn off signatures.
-Notification system and thread following features like in Reddit. "Notify me" uses email, which is simply antiquated. I don't want junk in my mailbox.
-The quote and hyperlink system is so bad, I rarely use it anymore. To quote someone, you should be able to just type @username >quote here, or press a button and a quote box appears.
-Since this is an engineering website, and engineers use drawings: a sketch feature.
None of these things are make or break by themselves, but I do find myself on other forums (not specifically engineering) because they're more usable. I mostly use Reddit. It has numerous flaws, but is nonetheless a pretty solid system. I don't think every forum has to be non-linear like Reddit; it could be linear, and still have a lot of usability.
If the new owners really want to rake in some cash, that's the way to do it. I'm a business owner. I'm not using the same calculation standards I was using 1 year ago. I'm always improving, lest I get left behind.
Regarding the argument about liking the old-school style of the website: I get it, it's a personal preference of mine too aesthetically. I don't necessarily need it to look better, though it can definitely use less clutter. I'm mostly talking about usability. But anyway, I'll refer to Reddit again in this regard: there's a Classic and New look that you can easily switch on that site. I always use classic. The young folks probably don't.