Depends on what the blank book is mostly. I mean drawing an Optimus on a Drift Blank cover might be ok for a personalized sketch, but a Drift sketch on a Drift book seems a little more logical.
I would say a character or scene from the book would be best, but even some of the RI covers show a scene that has 0 to do with the book. Or your Favorite bot.
The 2 Sketch MTMTE #1 1:100's I have were done by Burcham (Colorist). Tappet and Jhiaxus were the characters I got randomly, and it is only a small head picture, not a huge page spread. They had a few, but IDW sold out long ago.
If I had some of the newer Sub Variants (Issue #46 for both ongoings) IDK, Tarn and Megatron might be good fits for the MTMTE cover, Fort Max was on the Artist Editions RI and would make a great sketch, and Optimus vs. Galvatron seems to be the ongoing (Formerly RID) theme. Arcee and the other new Fem-Bots would be a good one as well.
We also have a new All Female Combiner showing to be a little popular.
You bring a very valid point. It seems to make a lot of sense. Much more sense than I have been doing in practice.
My sketch covers have all been 80s movie themes. A re-imagining of a movie poster or movie scene with G1 characters. Every time I take a request like that to an artist at a con or through the mail they sound very excited to work on it. I supposed they could be just acting nice. But it makes me happy to see that they are acting excited to do a commission like that.