If you apply one system with an index filter to all markets with a big watchlist, you will be turning that system on or off for all markets simultaneously. So, do you want to build country diversification into your portfolio intentionally? If you’re going to build country diversification intentionally, then applying the system to all markets simultaneously won’t force country diversification. You might get all of the signals from France this week and they might be the highest ranked. But if you have different systems on France, German, or Swiss stocks, then you might get a different mix.
There are pros and cons each way, and you need to test each market individually to see whether the system holds up and performs well in each market. Because if you dump it all together into a watchlist and it works, you’ll get complacent. But if you test it on one stock market and subset, you may find that it’s terrible and the rest are good. So I suggest doing a bit of investigation first rather than deciding. Do the investigation and see what the analysis tells you.