We do have an impressive amount of type assertions.
I've tried to lookup the .net type hierarchy and keywords to see what we might be missing.
See the bottom for types and keywords.

Here's a rough sketch of what such APIs could look like, but please notice that I haven't checked if they are:

• even possible
• desirable.

• ClassAssertions : TypeAssertions
• ClassSelector : TypeSelector
• EventInfoAssertions : MemberInfoAssertions<EventInfo, EventInfoAssertions>
• FieldInfoAssertions : MemberInfoAssertions<FieldInfo, FieldInfoAssertions>
  • [Not]BeVolatile
  • [Not]BeConst
• FieldInfoSelector
  • ThatAre[Not]Volatile
  • ThatAre[Not]Const
• InterfaceAssertions : TypeAssertions
• InterfaceSelector : TypeSelector
• MemberInfoAssertions
  • [Not]HaveAccessModifier
  • [Not]BeOverridable
  • [Not]BeStatic
  • [Not]BeAbstract
• MethodInfoSelector
  • ThatAre[Not]Async Adding ThatAreAsync() and ThatAreNotAsync() to MethodInfoSelector #1725
  • ThatAre[Not]Overridable
  • ThatAre[Not]Virtual Adds ThatAre(Not)Virtual to MethodInfoSelector #1744
  • ThatAre[Not]Static Adds ThatAreStatic and ThatAreNotStatic to MethodInfoSelector #1740
  • ThatAre[Not]Abstract Add ThatAre[Not]Abstract to MethodInfoSelector #2060
  • ThatAre[Not]Sealed
• PropertyInfoSelector
  • NotBeWritable PropertyInfoSelectorAssertions.NotBeWritable? #1256
  • ThatAre[Not]Overridable
  • ThatAre[Not]Virtual Add ThatAre[Not]Abstract, ThatAre[Not]Static and ThatAre[Not]Virtual to PropertyInfoSelector #2054
  • ThatAre[Not]Static Add ThatAre[Not]Abstract, ThatAre[Not]Static and ThatAre[Not]Virtual to PropertyInfoSelector #2054
  • ThatAre[Not]Abstract Add ThatAre[Not]Abstract, ThatAre[Not]Static and ThatAre[Not]Virtual to PropertyInfoSelector #2054
  • ThatAre[Not]Sealed
• StructAssertions : TypeAssertions
• StructSelector : TypeSelector
• TypeAssertions
  • [Not]BeStatic (Not)Be(Abstract|Sealed|Static) class assertions implemented #796
  • [Not]BeAbstract (Not)Be(Abstract|Sealed|Static) class assertions implemented #796
  • [Not]BeSealed (Not)Be(Abstract|Sealed|Static) class assertions implemented #796
• TypeSelector
  • ThatAre[Not]Virtual
  • ThatAre[Not]Static Add several new methods to Types and Methods Assertions #1306
  • ThatAre[Not]Abstract Add ThatAre[Not]Abstract to TypeSelector #2058
  • ThatAre[Not]Sealed Add ThatAre[Not]Sealed to TypeSelector.cs #2059
  • ThatAre[Not]Classes Add several new methods to Types and Methods Assertions #1306
  • ThatAre[Not]Structs Add ThatAre[Not]Structs in TypeSelector.cs #2056
  • ThatAre[Not]Interfaces Add ThatAre[Not]Interfaces to TypeSelector.cs #2057
• TypeSelectorAssertions
  • [Not]BeSealed Add several new methods to Types and Methods Assertions #1306

References:

Type hierachy:
https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/members

• Members
  • Fields
  • Constants
  • Properties
  • Methods
  • Events
  • Operators
  • Indexers
  • Constructors
  • Finalizers
• Classes
• Structs
• Interfaces
• StaticConstructors

Keywords and which type they apply to:
https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/

• member, type
  • access modifiers:
    • private
    • protected
    • public
    • internal
    • protected Internal
• classes, fields, methods, properties, operators, events, and constructors
  • static
• method, property, indexer, or event
  • virtual
  • override
• member
  • new
• type
  • abstract
  • unsafe
• method
  • extern
  • async
• class
  • sealed
• class, struct, interface, method
  • partial
• field
  • volatile
  • readonly
  • const