Here is the thing, though we want to be wishful, "we" are the "everyone". 100% compatibility isn't possible. What you say falls on that, anyone not using tag cannot use the buttons? You get what I mean right? Just how you can adjust the keys in the config.
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(int)k_t::UP,
(int)k_t::DOWN,
(int)k_t::LEFT,
(int)k_t::RIGHT,
(int)k_t::z,
(int)k_t::x,
(int)k_t::c,
(int)k_t::a,
(int)k_t::s,
(int)k_t::d,
(int)k_t::new,
(int)k_t::new,
(int)k_t::RETURN);
else:
in.new[n].set(0, -7, -8, -5, -6, 0, 1, 4, 2, 3, 5, 7);
Add004 isn't only a tag system, that is the huge misconception. His plan, keep the features without having to use the life-bars themselves. You plan to replicate all those other features as well? Really it's just more work for yourself, but of course, do whatever you want.
A lot of these things you suggest should be done by the users that want them. Tag systems vary for example, you take away the ability to use one over the other, or it means you have to build around it.
For the general user, I'd think general capability should do the trick. Hopefully you don't overwhelm yourself trying to make a game out of an engine.