Distracted by Bright and Shiny Things

I'm so close to finishing the revisions on Honor Bound that I can finally see the light at the end of the tunnel. Now that I'm nearly done, I've pulled out the first draft and notes for the second book in the series, tentatively titled Spell Bound. I'd written both Honor Bound and Spell Bound one right after the other without doing any revisions. I haven't looked at anything, just complied the notes by chapter and printed out the manuscript. Both are sitting on the corner of the credenza waiting patiently for their turn.

The problem is I keep having ideas for scenes and dialogue pop in my head for a book further in in the series. A book I don't even have fully formed yet in terms of the hero and heroine's goals, motivation, and conflict(s). This story wasn't planned to be written until at least the third or fourth book was completed. (I haven't decided if it would fit better into the overall arc as the fourth or fifth and most likely final book in the series.)

The picture below describes where my brain is right now: