Having experienced this rule in PF2, I don’t like this idea at all. Furthermore, in my experience, it doesn’t widen the design space, it shrinks it.
First off, if implemented, the designers must commit to a laser-like focus on the possible bonuses on attacks. The game just isn’t as fun if your character has a 5% chance to crit while another character has a 20% chance.
“But,” you may ask, “ isn’t it already the case that all players who max their main stat have the same attack bonus?” Ah, but you have to consider circumstantial bonuses and how easy it is for each class to get them. And you’d better ensure that all characters get magical weapons at the same time (or not at all), because no one wants to be the only character who is critting rarely in the party.
Second, well I “second” the comment about additional arithmetic slowing down the game. In 5e, if you roll a 15-19, you pretty much know you hit, no arithmetic needed. On a 1-5, you pretty much know you missed (barring oozes and zombies). Anything else, you’re doing the arithmetic, and some people just aren’t as quick as others. With 4 players, the extra arithmetic is tolerable, but with a large group (or a DM that is slow), it can quickly become unbearable.
Finally, the flipside. If beating AC by 10 or more is a crit, the range of character ACs narrows dramatically. In my PF2 game, I quickly realized that 3 of the 5 characters had the same AC. The leather-armored rogue? The same AC as the heavy-armor 2-weapon fighter. Last place? The wizard in cloth (with a +1 due to Mage Armor) at 3 AC less than either of them. This ruins verisimilitude for some players, and reinforces the “gamist” feel of the system for others.
TLDR: a +10 crit rule (or a -10 crit fail rule) ensures that the DM has to pay a lot more attention to keep bonuses and ACs within a tight range to ensure that certain members of the party aren’t overshadowed. This tight range can restrict design space and can stretch verisimilitude when a wide range of concepts end up having similar mechanics.