How about spreading some Pity Shield Monitors in the Singleplayer Stages like in Sonic Mania?
I believe the Pity Shield is only intended to help players who are being killed a lot in the ring-slinger game types (such as Match). It's directly worse than all other shields (except the Force shield if it only has 1 hit left,
I think), where-as the other shields are different from each other, not directly better/worse. It would just not be fun to encounter it as a monitor in a level, let alone accidentally popping it if you have a different shield and don't want a directly worse shield. In other words, I just personally don't think that's a good idea.
Edit: Also, sure, the basic shield is in Sonic Mania... but if you look at Sonic 3 and/or Knuckles, there's not a single basic shield, with there only being the different Bubble, Electric, and Flame shields. That it was taken out of that game (compared to Sonic 1/2) helps show that having a basic shield in the mix isn't fun, since it otherwise likely would have remained. You have to remember, Sonic Mania is... sort of a "remix" of classic Sonic games, including 1 and 2 (with their basic shields).
One thing that might help it fit better in stages would be making it so the Pity Shield monitor won't give you a shield if you already have a different one... but the concept of [a monitor doing different things depending on whether you have another shield or not] sounds awful to me.
Edit 2: And another thing, in Sonic Mania, Sonic has a Drop Dash ability. If you have the Bubble, Electric, or Flame shield, Sonic's mid-air ability changes, but as one might like to combine protection from damage with the Drop Dash ability, the basic shield allows you to do that. I think that may also be one of the reasons it's there in Sonic Mania.