I think creating a responsive template is the most important thing you can do. Something clean, concise and applicable for a variety of needs will probably serve you best. (IE--your first suggestion). Content is tends to what make people convert, although the ability to add header images could be beneficial. This could also be a chance for you to A/B test a handful of templates. Create a few more basic ones, and follow up with some more elaborate ones and see which converts better. However, I've just tended to find that subject lines/content assist with conversions more so than template design. Using all the features that marketo offers--snippets, segmentations, dynamic content, tokens, RCE modeler stages--will all enable you to send you the perfect message at the perfect time. Do you know about templates.marketo.com? There are a bunch of free templates on there.
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