[b]Steve of LetsRun replied to me that:
. . . It's an interesting idea, but not one I see happening in the near future for a couple reasons. First, the IOC is generally looking for ways to cut or maintain the number of athletes at the Olympics, not add them. That's why there has been talking in the past about cutting certain track events. Another reason you said yourself: "By limiting Ethiopia and Kenya to three athletes apiece." I don't know if the rest of the world isn't going to support a decision that is going to turn the Olympic Marathon into the "East African Championships." But the World Championships Marathon used to have 5 people per country, so who knows, it could happen. I feel like that would be more realistic, but wouldn't exactly accomplish your goal since many of the world's best marathoners don't race Worlds.
All that said, I truly think that the London Marathon every year pretty much accomplishes what you're going for with all the world's best men and women in one place at one time. Yes, there are some that miss it to run Boston or another spring marathon, but they are in the minority and often people who wouldn't contend in London anyway. London is so deep it's actually a more competitive and harder to win marathon than the Olympics.
Toni, and I believe others, suggested (dreamt) somehow getting Boston and London to alternate recruiting top female/ male marathoners each year as way to avoid diluting the fields at the competitive ends as they generally are.
After Rio, many were excited to see top 10 beyond just the marathon winners, and nearly every conversation started "if the marathon was scored as is cross country, we'd have placed X . . ." Understandably for the reasons Steve outlined above, this isn't terribly likely to implement in the Olympics, especially when we inexplicably haven't seen x country raced in those games in years, but, and this would add athletes to transport, clothe, transport, etc, can we hope to see a couple more athletes added to the WC and see x country style scoring added? This could also be a matter of if you build it they will come.
LetsRun: "Where ... Dreams Become Reality" - Let's make this happen.
How to even begin approaching making this suggestion is above my pay grade, but I thought I'd pass along the sentiment.