Having run older versions of D&D I was a little taken aback by the concentration of spells that will require concentration. I am just talking about spells with the mechanic right now but may address other concentration issues later. As it stands way too many spells have this and not to mention the wizard has been neutered beyond what he should be. Bam hit well the one spell that i can cast to help that needs concentration ends because of con save fail. And only one can be cast that reqires concentration which does not leave many options on your list, no wonder i don't have a single mage in my game group. So this is what I propose i did some checking and there are 10 rounds in a minute and most spells that require concentration last that long max. So lets say you cast a spell that requires concentration but opt for not concentrating then I would say the duration would be haf of base which would be 5 rounds. now in most of my old games I ran most combat does not go past a few rounds let alone 5 and if they do the other side is hurting pretty good. So I think this would be a good work around for the caster in the group and give a bit of the old feel to how the caster was well more useful. Or you could make the concentration a mental thing and a wisdom save as being a marine most things are the will and deternination to focus and keep alive is inpart a mental thing as much as physical.