Actual ex-copyright/patent attorney here. Have worked on board games and copyright issues with apps vs content creators (I crushed many dreams or thieves, depending on POV). However,
I am not your attorney.
I would have charged you about USD2000-5000 for advice and assessment, 10k per year to see your product published.
This is a long post as it includes deliberate language. I have tried to break it up into smaller sections
You cannot violate Wizards copyright.
Any text included in a published source book is not allowed. If you are found out, especially if you are making money, you will receive one of (1) a polite take-down notice (this seems to occur), (2) a C&D, (3) a DMCA notice, (4) a civil suit variant of some type. Any property you create based on Wizards material, will be owned by them, plus can included punitive damages (approx. 4x what you made, plus their Hasbro lawyer costs).
However, you mention the OGL (also include various SRD)
Titles are not protected by copyright. Names are not protected by copyright. Lots of written text isn't protected by copyright. However, most of it is.
The doctrine of merger says that where a rule has only a handful of useful ways of being expressed then where the expression is little more than the writing down of the method without substantive creativity in expression, then even the written form of simple rules cannot be protected by copyright.
Most (but not all) of the 5e mechanics, and even the names, are a variation of the OGL, hence, free to use (with attribution). E.g. spells have distance, chance-to-hit, duration, damage, elemental effects, status effects. All spell mechanics are just variations of the first written spell.
Anything in the OGL, SRD or
another games SRD is open to use. See
http://www.5esrd.com/home for an example of multiple sources combined to re-create much of D&D 5e.
Other modes of publishing
If the product you create is
available for free, you get more leeway in regards to "fan art" and "derivative works". At worst, you get a polite email and/or take-down notice.
Certain perks are allowed under the conditions of the Wizards Fan site policy. However, they will be more restrictive in regards to implementing Wizards material via third parties (such as re-building 5e with OGL/SRD+3rd party games).
I have read that people who approach Wizards with a created property will often be instructed into what editing is required (don't expect a quick response). As opposed to putting your project on the app store, where a single rule violation gets the whole project removed.
Summary
You can use all:
- race titles
- class titles
- sub-class titles
- ability titles
- spell titles
- background titles and mechanics
- item names
- mathematical equations
- game rules found in freely licensed material (most combat, ability checks, spells mechanics, class/racial/background/feat modifiers, basically "the game")
- the names 5e, fifth edition, or variants thereof
You can use some:
- your own flavor text
- substitution of your own alternate text for copyright text
- other games freely licensed material where attributed correctly (e.g. Pathfinder)
- spell mechanics (most fall under doctrine of merger)
You cannot use:
- background features or descriptions
- feat text (excluding OGL/SRD)
- other text not included in the OGL/SRD, including spell effects
- D&D, and variations thereof
- an ampersand (&) in your game title, subtitle in a way that resembles D&D
- images you do not have permission to use
- music and sound effects you do not have permission to use
- a format that could be considered "passing off", so no colours, fonts, non-textual element arrangements including break-out-boxes, backgrounds or schema as found in the books
Final point
At any time, Wizards LLC can attack you, even if you are in the right. At that point you will need to defend yourself. They will be very nice and very apologetic, however, it is usually a matter of whomever has the most money.
I still say make your character creator, but just aim to be conservative in what you include.
