There are no “do-it-all” plugins available, since the improvement settings so strongly depend on the (quality of the) input material and desired results. The 8mm material was good and satisfactory in the past, but it will never get to the quality of nowadays technology, whatever we do.
(Have a look at the pretty good attempts here: https://www.youtube.com/watch?v=hZ1OgQL9_Cw)
The basic material should be good so I hope that your mp4 scans are as good as they can be. Depending on the scanning firm and methodology (like scratch removal, color- and white balance corrections) there are a little or a lot of work left. With PD you can do reasonable cleaning and color corrections, and you will probable see that these corrections will differ from scene to scene.
In addition, you may also want to look at the glueing at the splices which tend to be darker, cut them out and redo them now with digital "clueing", and maybe even with new transitions. The removal of unsharp (unwanted) scenes will also improve the result. And maybe you want to reorder the scenes a bit too. It is all so much easier now.
And look at producing in HD quality where possible.
Also look at the recent discussion here:
https://forum.cyberlink.com/forum/posts/list/83345.page
It will take some time to get things right. But the audience will love it.
Have fun.

I took a look at the website of the conversion service you used, and can find no mention of their process to transfer 8mm film to video. There are several methods of transfer that produce different quality levels. Many companies, especially those who claim high quality, describe how the film is transferred. And some offer a test service; send a short piece of film and they will digitize it for you before you place an order. If you don't think your existing transfer is up to snuff, you may want to try a test transfer from another company.

Quote Hi there!
I'm putting together a home video for my parents, from their old 8 mm films that were converted to mp4 through imemories.com.
The quality is pretty terrible, I realize it's not going to be great, but does anyone have any suggestions on how to improve quality. Any plug-ins that may be helpful?
Thank you!

When I've worked with older videos my problem has usually been noise, lighting, and color. I usually use some denoising, brighten up the image, and adjust the color, since a lot of my older videos have the color washed out. So using "Color Enhance" or adjusting temperature and/or tint.

Problem is you have to be careful with lighting...if I brighten my video too much it introduces more noise, which defeats the purpose. So personally I'd rather have a darker video with less noise than a brighter fuzzy video. You have to play around to see what works. Unfortunately there is no one-click button to restore old videos.