I have mountain bikes that I have tweaked for rail trail comfort biking, and they work pretty well.

If you raise the bars you may need to also replace cable and housing, but I never know until after I raise them. I would say that more than half the time there is enough slack in the cables and routing to let me make the mods without increasing cable length.

My suggestions of things to consider are raising the bar, getting a bar with some sweep (and rise) to it, and in addition, I have become quite a fan of Ergon (or similar) grips. And of course, whatever tires make sensse for your riding, I prefer semi-slick with some tread due to the gravel/crushed stone rail trails in my area.