When I want to study and replicate the latest best slide decks I search for things like "best slide presentations" "beautiful slide" "business presentations of 2017" "amazing slide deck" etc. in Images. From here I can see the slides I like and visit the site. Common destinations are groups on pinterest, deviant art, private companies that sell presentations and listicles that grouped nice presentations together.
Now let me stop and clarify that this is a search for linear rectangular slide shows like those made by PowerPoint or Google slides. There is a growing list of more dynamic presentation software out there like Prezi.
Now once you have some designs you like open PowerPoint, open a blank presentation, open master slide editor and create designs based on your inspiration. Don't be afraid to make graphics in other programs and import them.
Now let's step back for a minute. Is this a one-off presentation that you will create and they will use, or will they need to be able to edit it? Are you making them a template or a finished presentation?
If it's a template than you have to make several master slide layouts. Think Title slide, section header slide, 1 main graphic slide, image on right slide, image on left slide, and others as needed. make them very consistent and similar to each other, same colors fonts proportions etc. These will be the different pages of your presentation.
While we're talking about templates, it will be tempting to open their presentation and apply a template. I don't reccomend it. Only if the original presentation used all the proper titles and paragraphs and image holders there will be mess to clean up and then you forever have a file with 2 master slides and 2 sets of settings.
If it is not a template you don't have to mess with more than one master slide, and not even that if you want full flexibility in placement and graphic elements.
Tedious but good control: after your title slide make your first content slide, style it and fill with contents. On the left side slide previewer select the slide and "duplicate" slide. Go to the new one and enter new info, change graphics, perfect it, then duplicate again and repeat.
This is also tedious but when I'm making a new clean presentation from an old one I open both and recreate each slide in the new, Not copying and pasting because the formatting comes with the pasting.
So to answer your question, look out to the internet to see the latest design trends in slide presentation.
Its an evolving format and a moving target.
Don't forget: elegance, lots of white space, few words, large words, few bullets, bold clear graphics.