You shouldn't have any affect on the rankings, i've read cases where people had positive results but thats up to the individual case. What you are doing right now is serving one language content everywhere, if you want to add lets say German language and you are after displaying German content for Germany you should expect better bounce rate and CTR.
Warp up alternative language or region content benefits:
- Increase in indexation
- Bounce rate
Even if you dont get the hreflang correct you wont lose anything as google simply ignores it, but you want to make sure you wont serve the same content otherwise would be considered as duplicate.
To speed up the indexing process i would recommend creating a hreflang sitemap and submit it on Google console.
Short answer: As long as you serve the content in different language or currency whatever the case is you should have positive out-come