Proposal

Arrowheads can be solid, cross and point:

Now we add stick as a new option:

Unfortunately, plantUML and Mermaid has slightly different code dialects:

Mermaid use
->> for solid
-) for point
-> for no arrowhead

For the new arrow type we use -|

We also introduce half arrow for stick and solid

Option A

Solid half arrow top -\
Solid half arrow bottom -/
Stick half arrow top -\\
Stick half arrow bottom -//

Solid half arrow top reverse /-
Solid half arrow bottom reverse \-
Stick half arrow top reverse //-
Stick half arrow bottom reverse \\-

Alternatively Option B
Solid half arrow top -|\
Solid half arrow bottom -|/
Stick half arrow top -\\
Stick half arrow bottom -//

Solid half arrow top reverse /|-
Solid half arrow bottom reverse \|-
Stick half arrow top reverse //-
Stick half arrow bottom reverse \\-

We also introduce the ability to and a central connection circle at the end of the message:

This is achieved by adding an o after the arrow in the direction symbolizing the life-line like this:

Bob ->>o Alice
Bob o\\-- Alice

Other examples:

Bob -\ Alice
Bob \\- Alice
Bob //-- Alice

Bob ->>o Alice
Bob o\\-- Alice

Bob <-> Alice
Bob <<->>o Alice
Bob o<<->>o Alice