Adding the letter s (plural) to your brand name won't have any large effect on its registrability.
Imagine you wish to register a brand called "Lion" for a phone operator in the US. During your trademark research, you find there already is a brand called "Lion" offering the same services. If you alter your brand name to "Lions", it won't provide any aid in your situation.
Once your brand name is assessed by the US examiner, it can be refused since "Lion" and "Lions" can be very easily confused. Also, the owner of the brand "Lion" would have legal grounds to oppose your application based on the high similarity between the brands.
Similarly, in a situation where your trademark is too descriptive of your goods and service, adding the letter s (plural) wouldn't help to increase the uniqueness of your mark.