Art block and the lack ideas is a very common thing for an artist, there are many ideas and ways to help over come art block such as this fantastic article PE: Your Worst Enemy.
This article will be listing some ideas to start your own personal project and ideas for small collaborations.

What is a Personal Project?
A personal project is something undertaken by an individual to achieve a goal. A series of art pieces (visual or written) that are part of a theme or goal can tell a story. (Example: PE: Telling Stories with your Art). Equally a series of artworks or a theme can do the same.
Some common ideas are:

• Seasons (Summer, Autumn (Fall), Winter and Spring)
  
• Elements (Water, Fire, Air, Earth and optionally Aether (Spirit))
• Times of the Day (Dawn, Noon, Dusk and Night)
• Days of the Week or Months
• Western Zodiac (Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces)
• Chinese Zodiac (Rat, Ox, Tiger, Rabbit, Dragon, Snake, Horse, Goat, Monkey, Rooster, Dog and Pig)
• Seven Deadly Sins (Wrath, Greed, Sloth, Pride, Lust, Envy and Gluttony)
• Seven Heavenly Virtues (Humility, Charity, Kindness, Patience, Chastity, Temperance and Diligence)
• Family (Fathers & Sons, Mothers & Daughters, etc)
• Colours (Red, Blue, and Yellow; Green, Purple and Orange)
• Photostories (How to Create Photostories, Part 1)
• Prompt Banks (MarcoEmma's Prompt Bank)
  

These are just some ideas to get a project going, ideally a project should consist series of artworks to complete a goal, can always add additional items to your project as you go. When making the artworks create a summary of smaller versions that collect the project together and track your progress. If don't want to include the project summary in your gallery, use Sta.sh instead to update it.
Having a folder dedicated to the pieces of the project may also be useful.

Another suggestion is to use memes and other practice themes to advance your art. Maybe focus on things you don't typically do in your current art or things you wish to improve upon.
Some common ideas:

• Draw from Life (Still Life, Portraiture, Landscapes, etc)
• Expression Practice (Happy, Sad, Angry, Excited, Confused, etc)
• Anatomy Drills (Hands, Feet, Legs, Shoulders, Abs, etc)
• Ageing Practice (Baby, Child, Teen, Young Adult, Middle Aged, Old, etc)

The idea behind a practice series is to have a list of items to make an artwork of with a goal in mind. Like the personal project, track your progress by generating a thumbnail collection of the artworks in the series and maybe a folder in your gallery for it.

If you would like to work with other people, but are a little intimidated you can always start with a smaller collaboration with one or two other people. Make sure your goals have been set before starting, along with timelines so each of the participants know what they are getting into.
Common ideas for small collaborations are:

• Switch-around Meme (each member does a sketch, then linearts a different sketch and colours the third)
• Draw a different character each
• Connecting Scenes (same shape, no shapes or different shapes)
  
• Draw an item (tree, person, car, etc) to make a larger picture
• Create a group of characters together

The goal of the project needs to be clear, if there is a template the contributor needs to conform to make sure they have easy access to it. For a small collaboration anything overly complicated is going to be different to manage, but persistence and good communication will go a long way into making the collaboration a success. Any of the personal projects can also be adapted into a collaboration, but at least one person will need to manage the project as a whole.