I've talked to Dave Levy at Ti Cycles about a replaceable dropout on my next bike. So I could use either conventional road dropouts (probably shortish horizontal as that makes for really easy wheel changes) and the really long road style horizontal but with a front "L" shaped exit so wheel slides forward, then down. Really easy wheel flips for fix gear. It's what's on my avatar photo bike and after 13 years, well proven. These could also be mounted to adjust lengthwise for fine tuning chainstay length.

On my avatar bike, I do not notice the rather large (~6cm) wheelbase changes I make but that might be because they are dwarfed by the corresponding change from 24 tooth to 12 tooth cogs. What I do notice is rear wheels being too far back when I climb out of the saddle on hard hills if the pavement is slippery. That spinning rear wheel costs me energy! For really hard climbs, I want the chainstay length under 16". Since this usually leads to tire clearances or the need for extremely steep seat angle, next bike is getting an "S" shaped seattube. And a normal setback stock seatpost! With a seat that isn't slammed! A combo I have never seen on a bike that fits.