Sure in a larger sense, we can do whatever we damn well please. BUT, within what is commonly accepted, can one create their own reactions? It surely would require DM's acceptance, but is it something that is even done to begin with? Or is it accepted that we are restricted to only those things that specify that they are reactions? For example spells explicitly identified as reactions?

Unless your DM homebrews a reaction you can use, or you bring an idea for a reaction to them and they agree to it, you are pretty much stuck with what’s in the books. Like your example of a reaction spell like Shield

Yeah, Bonus Actions and Reactions are pretty much RAW exclusive; you can only use them for a printed ability or mechanic.

Cool. Kinda figured that. But you can't blame a guy for trying can you? 🤷‍♂️

While it's not explicitly labeled as such, contingency can turn another spell into a de facto reaction, which sounds like the kind of thing you're looking for

There is also the Ready action: https://www.dndbeyond.com/sources/phb/combat#Ready

This let's you use your action and reaction to do pretty much anything you can do with your action normally but outside your turn when your trigger condition is met.

Also if you are ever at a loss for what to do on your turn then generally speaking it is hard to go wrong with the Dodge action: https://www.dndbeyond.com/sources/phb/combat#Dodge

Quote from Fangeye >>

There is also the Ready action: https://www.dndbeyond.com/sources/phb/combat#Ready

This let's you use your action and reaction to do pretty much anything you can do with your action normally but outside your turn when your trigger condition is met.

Also if you are ever at a lose for what to do on your turn then generally speaking it is hard to go wrong with the Dodge action: https://www.dndbeyond.com/sources/phb/combat#Dodge

 
I was just about to post almost exactly this.

Yeah, take the Ready and then follow the advice for Improvising an Action (box below Use an Object) if there's anything you want to do that isn't listed as a normal combat action.

It's also worth remembering you get a free object interaction on your turn which you can use to do various things like open a door during your move, draw a weapon etc. (see Interaction With Objects Around You below Being Prone). It's not a proper bonus action, but there's a lot you can do with that that players often forget about.

Otherwise there isn't really any kind of improvised bonus action in Rules As Written. Your DM is of course free to allow an improvised action to only count as a bonus action if they feel it doesn't justify a full action (but isn't minor enough for the free object interaction) but that's purely a DM call, but then so is improvising actions in the first place.

You can take the Ready action to do most of the actions except those specifically taken on your turn.