I don't have an issue with a D8 sneak attack when not dual-wielding. However I would not like to see it based on weapon die.
Partly because I allow any weapon to deliver a sneak attack, but mostly because I like to see weapons like daggers used.
For my own campaign, hand crossbows will not exist and rogues (and bards) will not get proficiency in the rapier. Plus, Khaalis's Light Armor Fighter variant will be an option. A player wanting to be a corsair, duelist, swashbuckler, two-handed roguish fighter or other light/no armor fighter can take that. The Rogue will be for Assassin's, Thieves, and Swindlers that sneak attack with daggers or shortswords
Why no swashbuckler rogues or bards?
Feat killer’s instinct
Your sneak attack dice are now d8.
You can use your sneak attack once per turn provide you don’t have disadvantage. (As swashbuckler)
You can use sneak attack with any one handed weapon, including improvised one.
I thought of a crazy way to incentivize going for a single rapier/light crossbow: what if your sneak attack damage was equal to your weapon's damage die?With a quick calculation, a rapier/light crossbow rogue would deal 81-92% of the damage a dual shortsword rogue will tend to deal (without factoring in advantage; the rapier rogue would benefit more from advantage since their first attack would be less likely to hit and thus be in less need of a follow-up attack). They're still dealing less damage, which would offer a trade off for having their bonus action free for other things.How do you think this would play out? How would you abuse it as a player?
I looked this up so I might as well link it here: http://www.enworld.org/forum/showthread.php?364670-5E-Fighter-Variant-Light-Fighter-(experiment)For my own campaign, hand crossbows will not exist and rogues (and bards) will not get proficiency in the rapier. Plus, Khaalis's Light Armor Fighter variant will be an option.
Huh. I think we may have extremely different views on player agency to determine their own character concept.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.