Is there anything else after the english tree and reached level 25?
Doing the reverse tree seems like a great idea, because you are required to generate a lot more phrases and sentences in the language you are studying that way. I'm planning to do the reverse tree as soon as I've gone over the Spanish for English speakers tree two or three times so I feel like I really know it well.
Hi! Congratulations for reaching level 25. I've met several people who improved their English by watching English television shows and movies, listening to the radio, listening to music, etc. It would also be very helpful if you could find people who are fluent in English in real life and have a conversation with them. There are also many other websites for learning and practicing English separate from Duolingo. Have a good day!
The reverse tree is the opposite of your language tree. So, you would do the Spanish for English speakers course.
Also, you could learn French (or any other available language) for English speakers.