To this day, i think it's a shame we never got more Eccleston episodes. I would have liked to have seen his character developed over at least one more series. I thought there was a lot more milage in the concept of Eccleston's Doctor recovering from his involvement in the Time War. I really don't rate the whole War Doctor thing personally.
If there ever became a day that he would love to play the doctor again (and he looked similar to how he was during his tenure) then surely they could adapt to create new adventures which would fit into his timeline but we're not episodes at the time.
I've always felt this would be possible, not strictly just for Eccleston although I would love to see that.
I've actually always had an idea that if the series began to under-perform they'd invite an old Doctor back, for example the most popular David Tennant, and do at least another series with them and slot it into their timeline easily. This would allow for a change of scenery without having the current Doctor regenerate and change the show effectively.
What they could actually do if they still wanted to run both the current Doctor's series plus an old ones would be to have one run from say February through April and then the current Doc's series in the time it now comes on August through November. A part of me would love this because I'd love another "spin-off". I can, however, accept it is extremely unlikely due to money and the fact people would claim that younger viewers would get confused, which even though they seemingly understand the current crazy, alien concept, is to a degree is understandable.
Carrying on from this, we know that multi-doctor stories are possible and thanks to Time Heist we know that if one of them happens to have aged considerable - as Davison had - they can just say "ah that's just because there is two of us". I feel that is the beauty of Doctor Who, no one knows what can and can't be done to a vast extent. They could have a series staring all the alive Doctors and be like "yeah, well a thing happened" and no one could say "yes but X, Y and Z don't allow that".
To come back to your point, I would seriously love Eccelston to come back for another series, but I doubt'll happen.
1) Eccelston has said he wouldn't ever do Doctor Who again.
2) When McGann returned for "Night of the Doctor" there was a massive cry from the fandom saying "give him a spin-off or something" which was hugely supported and it never happened. I don't think the money is there.