I have mixed 50/50 beef and bison and the results were stupendous (sorry just read the salmon post). It was much richer and more flavorful than beef alone. I think you could do the burgers as is but with 90% lean, your burgers have a high chance of being dry.
I would suggest mixing either 25% up to 50% bison/venison/elk and a nice fatty cut of beef or trimmed fat off brisket to get the numbers down close to the 80/20 meat/fat ratio.
I agree, add some fat or blend with a fatty cut of meat. During deer season I routinely blend equal weights venison and prime brisket when making ground game meat.
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