I’m working on a theme but I was wondering: what type of footer widgets would you like to see implemented? And how many should there be in the footer (by default)?
It really depends on what the site is man. Care to explain? 
My bad. It’s a portfolio theme, currently I have:
Sitemap Categories Social Media Contact (text or contact form) Latest tweets
Any more?
Maybe a search bar and the nav aswell? Unless by Site Map you mean navigation.
Search is a good idea! And yes, I did mean navigation with sitemap. 
No worries 
Something I like to see in a footer is a mini contact form, you can really make a footer stand out with well designed form elements.
As it’s a portfolio site, how about a few people or sites you follow/visit which give you inspiration etc?
- Author had a File in an Envato Bundle
- Author was Featured
- Bought between 1 and 9 items
- Europe
- Exclusive Author
- Has been a member for 2-3 years
- Item was Featured
- Referred between 100 and 199 users
mini-gallery 
mini blog
like a blog, in your footer. Like completely different from the portfolio part. A separate blog, in the footer. Best idea ever.
- Author had a File in an Envato Bundle
- Author was Featured
- Bought between 1 and 9 items
- Europe
- Exclusive Author
- Has been a member for 2-3 years
- Item was Featured
- Referred between 100 and 199 users
mini blog like a blog, in your footer. Like completely different from the portfolio part. A separate blog, in the footer. Best idea ever.
you can do this with my theme :)) in the sidebar
