I have 3 suggestions for new blocks in a future update. The first one, is a When Block. It is "When: (blank) lines have been drawn". So, if you have two leave a trail blocks in your code somewhere, you make a new rule with "when: 2 lines have been drawn" and do move forward after that, and you play it, once both leave a trail blocks have been activated, the item you put the move forward block in, will move forward.
This is my next suggestion. It's a purple block that belongs in the "drawing" tab. It's the "bring trail to front" block. Pretty easy, if you put "when 1 line have been drawn, bring trail to front." You will see that once the line has been completely drawn, it will go infront of any characters or text objects that were infrount of the line. So, it's pretty much a "bring to front" block for trails.
My last suggestion is pretty much the opposite of the previous one. It is a purple "bring trail to back" block. I bet you know what it does.