I’ve seriously looked into this a couple of times (see my most recent thread).
If I were to go for it, then a hardtail with two sets of wheels would fit the bill. Things I learnt along the way:
– You may already own that bike (I used to do everything all on one bike back in the day);
– 26er would be fine, as 700c / 29er wheels with slicks are the same circumference;
– Rigid fork would probably stay on the whole time, as to change regularly is a faff.
I didn’t go ahead because I cycle to a busy railway station once a week at least and have to lock my bike up there. I also took a good look at my riding stats for the past year and saw that I do 85% of my miles on the road. So, I kept my Cotic Soul and brought my roadie up-to-date with a Cotic Escapade frame and some new 105 gear.
I’d still consider something like a rigid Surly Karate Monkey or Cotic Solaris as the ultimate do-it-all bike when the Soul needs replacing. But I’d go into it with my eyes wide open to the compromises.