My current back gear has a shifter without a gear number display, with only the + and - signs on the ends.
I was thinking to replace it with a numbered one. Would using a numbered one cause any issue? Do you have an idea? thanks
There are two things that you need to check to be sure that the shifter will work. The shifter needs to have the same number of gears as your bike. If you have 7 gears on the back, then don't go buying a 10-speed shifter. The second thing is that the pull-ratio of the shifter has to match the derailleur. This is why not all shifters with the same number of gears are interchangeable. This information can be found online or you can update your question with details of the derailleur on your bike and the shifter that you're looking at.
If you want to be creative, consider making something like this:
That's a 9 speed Dura-ace inline visual gear display, which is intended to sit inboard of the STI brifter. The gear inner cable passes through the clear view-tube, and rider can see the red marker move left and right as the cable moves. There are graduated lines to help see where your marker is.
Its essentially an extension to the outer cable of your gear line, and would need to resist shifting forces without collapsing.