Esperanto is a neutral language. I've discovered it when I was searching for guidelines in building a conlang.
While learning about the grammar of Esperanto, I was thinking if I could use that grammar, modify it, and create my own language. The reason to this is because both of my languages Croatian and English aren't enough sometimes to express what I want to say; and by Murphy's Law, when there's an arguing or a debate, I get misunderstood due to the weakness of a language's grammar.

My language would have:

• A little changed alphabet that uses only English letters, has Croatian sounds ČĆŽŠĐ, a blank "errr" sound as a suffix to solve the issue of suffixes merging with the original words, Japanese/Esperanto "w/ux" sounds, j=y jx=j replacement so that English speakers don't say [jx] every time they see "j" which is like Romanji "y".
  
• More verb forms like Perfect in English.
  
• "You go here and you do this" where it's more like a narrative/suggestive/slightly_imperative present tense.
  
• "I go to work every day" where the verb says what is usually done as well as in "I used to go to work every day" and "I will use to go to work every day" or "I would have use to go to work every day". Suffixes wouldn't be just -as, -is, -os and -us. Modal verb + -as/-is/-os/-us, then original verb + -es would solve this problem.
  
• Persons (1st, 2nd, 3rd) singular, plural,
  
• 4 grammar genders: Male, female, mixed/relative/undefined, unalive
  
• Pronouns and verbs with declensions for all persons, singular and plural, mixed gender plural and subject/object reference where there's to much "He told him that he stumbled and fell on his head which was on his and his and his.... URGH! So many he-s!". -am would be present 1st person singular, -asx would be present 2nd person singular, -aa would be present 3rd person singular, and so on.
  
• More sub-sentence connectors. English has "that" which has multiple meanings. "I say that that is wrong." The second "that" is accented, but how am I supposed to know when is which one accented? Why not have a separate word for a connector like that?
  
• Adjectives and pronouns with suffixes depending on the gender.
  
• Different case declension suffixes. -h for accusative, -g for genitive, -m for dative, -d for locative, -w for instrumental,...
  
• Ablative case for assignments containing "esti" verb
  
• A new case for "Created by *noun with this new case*" passive speech

Now, would this be a good thing? I don't want to let this make Esperanto look bad. At the same time, I'd like to develop a language for a video game, my stories, my friends and such which would later on become a language everyone would learn as they learn Esperanto.

I'd suggest to leave it to just your friends/stories/games. It's okay to create a conlang based on Esperanto, but I wouldn't suggest to propose practical use of it, unless you want fierce opposition and excommunication from the Esperantujo (look up the Ido language). On another note, once you're finished developing this conlang, could you show me the rules, phonetics, semantics, etc, of it. I'd love to look at it since I love conlangs

Zvoc47:
Now, would this be a good thing? I don't want to let this make Esperanto look bad. At the same time, I'd like to develop a language for a video game, my stories, my friends and such which would later on become a language everyone would learn as they learn Esperanto.

There are lots of language projects based in Esperanto. If your intention is not to mess or annoy Esperanto speakers with reforms, and just play, why not?

The only thing is that I think you have a wrong idea about Esperanto. Esperanto is not a game. It's a language in use. Yes some people learn it as a hobby the same they learn Klingon, Na'vi or Dothraki but Esperanto was created for the real life, not for hobby purposes and is used in the real life by its community of speakers, not just as a hobby. Even if I'm wrong about what you think, please understand that not every Esperanto speaker is looking forward to learn new conlangs, many of us have enough with Esperanto, and we want to keep it alive and useful. I'm not against other projects, I'm just against eternal proposers of new conlangs as the “right” option just because.

Ĝis, Novatago.

Conlanging is a great hobby and you even learn quite a lot about actual languages. You will reach many points where you will wonder: "Why is ABC like THIS in language X, but like THAT in language Y; and how will it be in my language?"

For instance look at which part is the subject and which is the object in these equivalent sentences:
I like dogs.
Me gustan los perros.

Do it! It can be great fun!

I will now comment on each of your bullets:

1 (Phonology, phonotactics, spelling):
- Define your phoneme inventory. Define where certain phonemes have allophones and how they are distributed (maybe the phoneme /r/ is realized as [r] in the onset, but [ʀ] in the coda).
- Define phontactics and stick to it and its restrictions. This is one of the weak points of Esperanto.
- Think of a nice way of spelling. Are there digraphs, trigraphs or not? Will you use diacritics or not? etc

2 (Conjugation)
Just fine. It is your language. Maybe you want to divide aspect from tense. Let's say vid'as for present tense and vid'iv'as for present perfect and vid'is for past tense and vid'iv'is for present perfect. Just a random idea. Which aspects? Perfective, imperfective, resultative, stative? Which tenses? Only one? Only two (past and non-past) like Germanic languages? A whole load of them?

3 (Mood)
Feel free to play around with the moods you find in real languages, or even create your own? Something crazy. Like "paenitive" if you are regretting what you are saying.

4 (Aspect again)
You can use modal/auxiliary verbs for this, you express this via inflection, via derivation, etc, etc.

5 (Personae and numeri)
The 3/2 set is quite common, however you are not limited to it. Will there be a forth person, or will you rely on reflexives? Will there be inclusives and exclusives? Will there be a dual, paucal, etc?

6 (Genera)
Sounds great.

7 (Inflection/marking/deixis)
Whatever rocks your boat.

8 (complementizer)
You are right. There is absolutely no need for a complementizer to look the same as a pronoun.

9 (agreement)
Only gender? Also number? Whatever you feel like.

10 (casus)
There are a zillion different cases, pick the ones you like best. Or maybe remove cases completely and directly mark syntactic roles without this clumbersome step?

11 (predicate noun over the subject)
Why even both with a word for "esti"?

12 (casus again)
Your choice.

Summary: Keep going and keep on updating us how the development goes.

I think it is fine but please don't call it Esperanto.

Zvoc47:While learning about the grammar of Esperanto, I was thinking if I could use that grammar, modify it, and create my own language.

People have been trying such experiments since Esperanto was first made public. However attempts at "reform" were never approved by the majority of Esperantists. You may find interesting Arika Okrent's In the land of invented languages.

It doesn't sound like you are talking about a reform of Esperanto, it looks like you're talking about making a new language with Esperanto as a base.

You should go ahead and do it, and enjoy yourself! Many speakers of Esperanto who make up their own language use Esperanto words or grammar as an inspiration for aspects of the language, so you should feel free to do the same.

Using Esperanto as a base for something new is a totally different thing from reforming Esperanto, and so long as it's put forward as a new language with an Esperanto base, no one really has a problem with that.

Aren't the joys of "conlanging" severely limited? A lot of work when the chance is almost certain that no-one will ever speak it, read or possibly even see it.

I realise this argument could have been (and likely was) made against Esperanto, but there was nothing else with really as much potential as Esperanto at the time, so a more realistic gap existed.

Even the most well-known conlang projects are ghost towns. There's no doubt some value in learning about language structures from 'doing', but I assume someone taking on such a job already understands how language structures work.

In my mind it just syphons away effort.

So, Zvoc47, all you have to do is devote your life to this project of yours. First create for yourself the vocabulary, syntax, morphology and phonetics. Then discipline yourself to several hours a day of translating great works of world literature into this new con lang! [I I always grin at this term, since 'con' in American refers to a person who is not honest,hehe.] Then create a style guide [like the Krestomatio] and have your father-in-law or some rich donor help you print or replicate and distribute [maybe by means of the Web] this language. After about 18 years you can call a world congress - there is a small beach front town in France called Boulogne-sur-Mer - they probably have enough hotel space for your first world congress. Maybe you should also work on a collection of 'sayings' in your new con language.
I wish you lots of luck. We'll get back to your success in this venture in about 20 years.

Zvoc47:
A little changed alphabet that uses only English letters

Since when those letters are English? They had been invented long before the English started using them.