Here are the examples from SRFI 243 translated to the proposed #?datum
notation. These look reasonable to me. AFAICT every aspect of the
problem is solved as well as or better than with the current usage of
#<...> and nothing is made worse.

Chez Scheme

#?void
#?procedure
#?(procedure +)
#?(procedure fold-left)
#?(environment *top*)
#?(input/output port stdin/out)

Chibi-Scheme

#?undef
#?eof
#?(procedure #f 0)
#?(procedure append 0+)
#?(opcode "write-char")
#?(Output-Port 140501393508544)
#?(Environment 140501393819456)

Chicken

#?unspecified
#?(procedure C_plus)
#?(procedure (?))
#?(procedure (scheme#write x . rest))
#?promise
#?(output port "(stdout)")
#?(environment interaction-environment)

Chicken eggs

#?(procedure (tween#quadratic-ease direction123 percent124))
#?cjson
#?sql-null-type
#?sdl2:surface
#?(sdl2:rect (10 20 300 400))
#?(mat 2x2
4.0 0.0
0.0 4.0)

Gambit

#?(procedure #2)
#?(procedure #3 fold)
#?(promise #6)
#?(output-port #2 "(stdout)")

Gauche

#?undef
#?eof
#?(closure (fold kons knil lis . more))
#?(subr (+ :rest args))
#?(subr "continuation")
#?(promise #x102097fa0)
#?(subr #?(<parameter> #x102ae8800))
#?(oport "(standard output)" #x100d3cc00)
#?(module user)

Guile

#?unspecified
#?eof
#?(procedure + (#:optional _ _ . _))
#?(continuation #x10716dae0)
#?(promise #?(procedure #x7f9b6980e6a0 at "<unknown port>:11:0" ()))
#?(output: file "/dev/ttys001")

Kawa

#?(procedure +)
#?(macro define)
#?(environment main)
#?(output-port "/dev/stdout")
#?(location-proc "gnu.mapping.ThreadLocation[param#1]")

Mosh

#?unspecified
#?(subr +)
#?(closure 62157904)
#?eof-object

Sagittarius

#?unspecified
#?eof
#?(subr + 0:1)
#?(closure #f 0:0)
#?(<promise> #x10d5200c0)
#?(<parameter> #x1061970a0)
#?(transcoded-port utf8-codec #?(buffered-port #?(binary-output-port
stdout)))