If you're keeping him purely to the Heresy period, perhaps something from the Purge of Nuceria might provide some inspiration - a two on one fight between the Word Bearers, World Eaters, and a smaller force of Ultramarines looking for vengeance (it's during this battle that three of the Primarchs - the templates for the space marines - meet in a massive brother vs. brother fight). Or as secretpaintgeek has suggested, some scuffles with the Alpha Legion. Most of their artwork tends to be very gothic and illuminated as Trystan pointed out.
If you want to bring it forward a bit into the more recent, 40k time frame - some of this pattern of dreadnought still exist in this time frame, though they are very rare (I believe that a chapter as proficient as the Ultramarines would probably be able to maintain a few that long) - then the Tyrannic wars seem as good a topic as any.
Other than those periods of history, the Ultramarines tend to have Roman sounding names and a great deal of their iconography draws from this also. Laurels, roman numerals, imperial eagle etc. They maintain a small empire of sorts (Ultramar - original no?) within the Imperium of Man itself, were the 13th legion to be founded and are pretty much the template all other chapters are meant to follow (though no ones seems to) being generally noble, skilled and proficient in all things military.
Ooorrrr, the chassis and/or occupant could be truly ancient and you could depict a true gamut of history ranging from 30k to 40k...which could be interesting.
Hope that helps.
Either way, a stunning paint job to date and the very best SENMM i have ever seen on a miniature - nothing even comes close.