You have some great independent thinking going on here. It may help to remember that people like to put things in categories because it makes us more efficient, but the natural world does not always fit neatly into our categories.

One category is the Kingdom Animalia. Euglena are not animals. They do move and eat, which are animal characteristics, but animals have to be made of multiple cells. Euglena belong to the Kingdom Protista. You are right that plants do photosynthesis, but so do other things, like seaweed.

Euglena have evolved over time, and may continue to change in the future, but no one can predict what the changes will be. It all depends on random mutations and how useful those mutations are in the changing environment. For example, let’s say that some Euglena were trapped in a cave. They couldn’t do photosynthesis without light. Chloroplasts and chlorophyll are expensive to make. So if a Euglena happened to have a mutation that caused it to make fewer chloroplasts, or none at all, it would probably do a lot better than the other Euglena. It could invest in other things, divide faster, and leave more copies of itself. Eventually, the entire population of cave Euglena might be without chloroplasts.

Being in the dark would not cause the mutation. Mutations are random. It’s just that being chloroplast-free would be an advantage in a cave. A mutation to be chloroplast-free in a place with light would be a disadvantage, so those Euglena would divide less than their competitors.

I don’t think they are likely to be very intelligent since they only have one cell, but I guess it depends on how you define intelligence. If you just mean that they can move and respond to their environment more than plants can, then they are already there.

What do you think intelligence means? When a plant faces the sun, is that behavior?

Great question. This is why classification of organisms can be so confusing. Linnaeus (who developed the Kingdom based classification system) based his observations on what was readily visible with the human eye and primarily organisms only found in his European homeland. As science expands trying to place living things into specific categories can become very difficult. One example is the Euglena. You can classify it as a plant because it has chloroplasts and photosynthesizes, but it does not have a cell wall which is another character that all plants have. But is can freely move around and it can take in food both animals characteristics.

So you can say that Euglena is a plant with animal-like characteristics or an animal with plant-like characteristics or you can place it in another category all together that only includes single-celled organisms. As for evolving into a animal/plant creature or loosing either plant or animal parts cannot be predicted. We can only look at the past and see that there has been little change in Euglena for a long time, so whatever system they have evolved seems to be working just fine. Guess we will just have to wait and see.

Euglena is neither plant nor animal. They're single-celled eukaryotes, and plants and animals both are multicellular.

Euglena's ancestor was a protozoan that enveloped a cell of a green alga and then began to use the entire cell as a chloroplast, in much the same way that green algae long ago enveloped photosynthetic bacteria, thus creating chloroplasts in the first place.