I don't agree with changing the name over to Surface. The Lumia name is a great choice, and they should continue to use it. I've thought about the product lines and offerings, and here are my thoughts:
1. Limit phone model numbers!
4xx, 5xx, 6xx, 7xx, 8xx, 9xx, 10xx, 13xx, and 15xx
Cut it down to 5 or 6, max!
5xx - entry level for developing countries
6xx - entry level for US/UK
8xx - mid level for US/UK
9xx - flagship for US/UK
10xx - flagship camera phone for US/UK
coin toss between keeping 7xx as another mid tier option, or sticking with just the 8xx series.
While they do and should encourage other device makers to support Win phones, there should be some base standards for the Lumia line to ensure smooth user experience and app availability across product lines. For Win10, 1 (ideally 2) gigs of RAM, minimum for all Lumia devices. FFC and flash on all devices. Dedicated camera button for mid-tier and up phones.
Have 6xx, 8xx, and 9xx product lines offer XL versions of devices. Release device(s) every 6 months, with a yearly refresh.
IE release 5xx, 6xx and 9xx at the same time. Then release 8xx and 10xx lines roughly 6 months later. Other WP manufacturers can release devices as they see fit, but the Lumia line desperately needs some kind of consistency in release cycles.
I'd also say start every product line at x50 to coincide with Win10 launch, so 550, 650, 850, 950, and 1050. From there only use x51 for yearly updates. So if the 550 was released in October of 2015, the 551 would be released in October of 2016. Then the following year, release the 560. No more carrier specific numbering schemes (ie 521, 822, or 928). Uniformity would be key (the iOS numbering scheme is simple and effective imo and would be nice to adopt something similar)