First and foremost, your developments should be about making your product more useable (efficient, effective and satisfactory). The big question though, is what will make it these things?
Here is a long list of things to consider:
Suitable materials
Methods of manufacture
Sizes
Ergonomics
Anthropometrics
Commercial viability and potential
The marketing features of your designs
Design optimisation
Design for Manufacture and Assembly (DFMA)
User-centred design
Inclusive/ exclusive design
User needs/ wants
DFMA
Components
Electronic aspects
Mechanical aspects
Programmed aspects
Materials
Composition/ assembly
Function success (tested)
Generative design
Biomimicry
SWOT
User feedback
Your master requirements
Manufacturing requirements/ options
Materials requirements/ options
Assembly requirements/ options
Component requirements/ options
Questions, Questions, Questions - what are you thinking?
Function
Aesthetics (research designer) / Affordance
Cost
Availability
Environmental Considerations
Material and component selection
Ergonomic considerations
Mathematic application
User needs and wants
Social, moral, ethical and cultural considerations
Manufacturing methods/ techniques
Scale of production
LCA
USP
Innovative
Makes a product useful
Aesthetic
Understandable
Unobtrusive
Honest
Long lasting
Thorough (in every detail)
Environmentally friendly
Minimal (the required level)
Function
Aesthetics
Cost
Availability
Environmental Considerations
Material and component selection
Ergonomic considerations
Mathematic application
User needs and wants
Social, moral, ethical and cultural
Manufacturing methods/ techniques
Scale of production
LCA
Health & Safety
Context
User/ target market
Materials
Components
Construction method
Functionality and performance
Ease of use
Ergonomics and anthropometrics
Other human factors (lifestyle impact)
Inclusivity
Fitness for purpose
Lifestyle impact
Trends/ styles
Marketing and branding
Getting a product to market
Who
What
When
Where
Why
HOW - HOW - HOW
Ease of use
Performance and function
Material suitability
Material selection
Construction/ manufacture suitability
Product impact
Value for money
Fitness for purpose
Changing consumer requirements
Throw away society
Biomimicry
Failure
Other experts
Innovative
Feasible
Sufficiently funded
Well researched
Ethical
Sustainable
Functional
Ergonomic
Usable
Well constructed
Appropriately manufactured
Made from suitable materials
Inclusive
Tested
Safe (meets ISO/CE/BSI)
Product Lifecycle
Life Cycle Assessment
Energy use
Single or multi material product or component