I've done at least six t-shirt quilts. The last three were memorial quilts for a friend. I did an all over meandering large stipple on one, a large meandering swirl on the second, and flames (for the firefighter in the family) for the third. I used variegated thread and multi colored thread on them. I had no problem quilting over the top of the silk screening designs on the t-shirts. They all turned out beautiful.

I would quilt it with red or maroon thread so that the red that is already in the quilt is linked and looks more balanced.

I have no suggestions for quilting, however you did a really great job on the top!

Another vote for wavy parallel lines down the length of the quilt. Start in the middle and work your way out to each side.

I would just do a basket weave (the first part of this tutorial) http://theinboxjaunt.com/2014/10/21/...uilt-tutorial/

You could also do it in each block, or across the entire quilt. I like to do each block (mostly because I find it easier), but it's totally up to you.

Yes, keep it simple. You don't want to detract from the t-shirts. I would tie it and then do something simple in the border like a meander. Distance for tying depends on the batting.
Well done!

I agree with Jratcliff. I would tie it -- especially for a picnic quilt.

My DD just did a allover stipple on this using clear thread.
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What is the color of your backing?

I agree that a simple meandering would set off the blocks very well.