Allowing the Slayer to use Dex OR Con for its bonus damage

[Garthanos]

I think that the essentials limiting you to using dex is supposed to prevent you from making an underpowered, 1-dimensional character. Forcing you to have dex secondary guarantees that you can use ranged weapons, acrobatics, have a good ref defense, have a good AC no matter what armor you pick, etc.

If you consider yourself a D&D expert, then switching out dex for any other stat should be fine: you're going to have to work harder to see the benefits of doing so, but hey! you're an expert!

One dimensional? hardly (STR/DEX seems more one dimension character wise)
I was considering a Detective Background / Wary Fighter with Focused Superiority (opportunity attacks get Wisdom bonus).. Perception when you already have good Athletics is better than acrobatics. (Flexibility in armor isnt necessary with Scale it is pretty darn good in the first place). I think the main loss for such a detective might just be the ranged combat (though it might be fine with Javelin throwing).

I dont think of it as they limited one to Dex they just designed it that way flexible stats exploiting is a Warlord class feature almost. (as you say DEX does cover a lot of bases).Where as having the secondary stat flexible could open opportunities for differing "characters" within the heavier armored less mobile better defended striker role.

To those who suggested a Feat Cost ? No I think charging a feat to do swap out as long as it was something other than their primary stat would seem to be charging for non improvement (Dex grants a lot)

Charisma seems to grant the least benefits though weirdly it would make good flavor. Perhaps if they had Spirited fighter (similar to Wary fighter but with different auxiliary benefits ) and other support. Diplomacy gets used a lot in our games. Charisma based skills are in the knights perview not the slayers... a background might help a little... but then Intimidation can really rock.

Multiclassing might be were the flexi-stat might be most useful... but how that works in the essentials world I am less certain about.