Only Roku could add a DVR ability in and I doubt they would do that as some of the companies providing free channels may not want them to do that. Besides 99% of the stuff out there is streaming on demand so a DVR is not needed. Sling TV is the only reason that I can think of where you want a DVR (other than Tablo) but that's up to Sling to provide a solution. Their main solution was to have most of the channels auto record up to 3 days back but not everything works that way.
Now for ESPN, keep in mind you don't really need a DVR for that. If you install the Watch ESPN channel you can view previous sporting events on demand with ought the need of a DVR. This is part of your Sling TV package.
Roku would never add a DVR feature to their boxes for streaming content or their channel count drops quickly.
Watch ESPN is great, I love it and finally have access to it with DirecTV, but DVR features aren’t available. I’d like to be able to rewind and fast forward when using it, but can’t.
Anyway, adding apps like Watch ESPN and then offering DVR features wouldn’t be up to Tablo at all, that would be 100% up to the channel owner to decide to implement if Tablo ever went the app route. To me, one of the most intriguing things about Tablo is that they chose to fit in with the devices that people already have and use instead of bolting Netflix and Hulu+ onto their interface.