I think something cool that could go along with this is a mod that will, at random, replace certain shields with a useable card, pulling from the original game. Not all cards would work, but here's some I can think of, all of which requiring Custom 1 to activate:
The Hierophant - Gives the player the blue ring powerup from Chaotix or Mania. If holding the Alabaster Box, it instead gives two charges.
The Moon - Gives the player a radar pointing to nearby Alabaster Boxes (or emblems or medals) for 30 seconds. When holding a box, it will instead point to any nearby monitors.
The Hermit - spawns a small shop to buy powerups or shields from using rings, similar to using a Turnover from the appropriate TBOI mod.
The High Priestess - Summons Mom's Foot to attack nearby enemies or bosses. If nothing is close enough, it will simply target the player, dealing damage but no knockback.
The Emperor - Simply skips the player to the boss of the current zone. (This will skip Metal Sonic's race entirely if used either during it or in ERZ2, will reset the player if used in BCZ3, and will outright not work in RVC or any challenge/extra levels.)
Death - Will immediately replicate the Armageddon ability upon use.
The Fool - Will teleport the player back to the starting position of their current level.
man, if there ever was a character from tboi added into srb2, like lets say the forgotten or DOGMA, it would be pretty cool
I think so, too, though even base Isaac could work. All shields, if not replaced with the mod I proposed the idea for above, will instead grant Isaac a random item from the original game. Most likely, only items from the original Flash game, but with any changes Repentance brought about. In terms of base moveset, he'd be around Amy's speed, his Spin ability is firing tears (autoaimed in third-person, manually aimed in first-person) and can crouchwalk with Custom 2. Holding Crouch and pressing spin would spawn a bomb, his only way to break literally any breakable walls. If granted flight, holding Jump would ascend, and Crouch would descend. Instead of rings, he'd grab Hearts and lose them how you'd expect (hearts are separate from rings and are earned from enemy kills and boss damage; red hearts are most common, soul hearts less so, then dark hearts, then holy hearts being the rarest and working like you expect. Rings essentially become coins in this sense.) Getting The Hierophant would give him two soul hearts immediately.
Custom 3 allows him to use the D6, rerolling any monitor he's immediately next to but requiring 6 enemy kills to recharge. Of course, getting an active item from a monitor will instead replace it with that item's abilities.