Yet another a bit "special" solution, but I would say it is not against the (current) rules, it's just a matter of interpretation.
1.
$ VII = 111 $
Roman $7$ = Binary $7$
2.
And another one...
$$ | -11 | = 11 $$ $$ abs(-11) = 11 $$
3. ...and another one...
$ II = 10 $
Roman $2$ = Binary $2$
4. ... and another "new" concept...
$$-iiii = -1$$ with the imaginary unit $i^2 = -1$ $$-i*i*i*i = -1$$
5. ...although the "game is over"...
...a new combination from known principles...
Can be interpreted as $|1| = |-1|$ or $|i| = |-i|$ or $|1| = |-i|$ or $|i| = |-1|$
6. ... and a slightly "odd" one ... (maybe I should stop now...)
$ XI = +11 $
Roman $11$ = $+11$
7. ... yet another unconventional "rot90" version...
Roman $2$ = 90° rotated $2$
8. ... another one, maybe a bit too nerdy or ... ?
$ 11 = 11 | 11 $
in many programming languages $|$ is a symbol for bitwise inclusive OR
so, this can be interpreted as
binary $3$ = binary $3$ OR binary $3$
or
decimal $11$ = decimal $11$ OR decimal $11$
9. ... another combination of principles from above...
$ 11 = \omega $
Binary $3$ = 90° clockwise rotated 3
10. ... sorry, again... a simple one which is not yet listed in any of the answers...
$ II - 1 = +1 $
Roman $2$ - $1$ = $+1$
11. ... and a last(?) one...
$ II + 1 = 11$
Roman $2$ + $1$ = Binary $3$
12. ... some other bases for new options...
(base 4) $11 = 5 $ (rotated by 90°)
$11_4 = 5 $
13. ... and combined with earlier ones...
$ VIII = 11 $ (base 7)
Roman $8 = 11_7 $
hmmm, I couldn't resist...
14.
$ 7 \wedge 11 = 1 $
$\wedge$ is used as binary exclusive OR (XOR)
$7_{10} \wedge 11_5 = 1$
$ 7_{10} \wedge 6_{10} = 1 $
$ 111_2 \wedge 110_2 = 1 $
15.
$ 71 - 1 = 11 $
$ 71_{10} - 1 = 11_{69} $ (base 69, ok very special)
16.
$ 11 - 7 = 11 $
$ 11_{10} - 7_{10} = 11_3 $
17.
... and finally(?) a nicer one...
(binary) $ 1111 = F$ (hexadecimal)
$1111_2 = F_{16}$
$15 = 15$
Now, I guess it is enough... unless somebody wants more.