There are many one-handed keyboards, and several websites devoted to them, such as
http://www.onehandedkeyboard.com/. I actually considered getting a Frog Pad once. To answer the original question, I'd like to see an eight-way directional pad controlling an onscreen virtual keyboard that's a three-by-three grid of three-by-three squares. You could type using just a thumb.
a_b_c_____j_k_l_____s_t_u
d_e_f_____m_n_o_____v_w_x
g_h_i_____p_q_r_____y_z_.
1_2_3_____,_?_'_____ó_ô_ò
4_5_6_____BS_Sh_!___õ_ö_ø
7_8_9_____0_(_)_____ŏ_ō_ő
And so on (you get the general idea), where Sh here means Shift and BS means Backspace and the bottom triad is not shown). You start in the center square of the center block, on Shift. Press any direction key and you move to the center square of the adjoining block (unless you press Center to stay in the current block).
That means your standard letter is two key presses:
lowercase j is north-northwest. Capitalize by first hitting the middle key: center-northeast-center gives
uppercase W. I'm showing an accented letter O set here, because it's clearer than having naked diacritics, which they'd likely be onscreen. In any case, you can see that an accented lowercase letter is four key presses:
long u would be east-south-northeast-northeast. (Two keys select an accent, then it puts you back at center-center to pick the letter. You can see some built-in awareness here.)
More accents and characters in the bottom trio of grids. You can modify the grids; I think you can have one for shortcuts, so if you use
é often, you could set it as a normal two-key instead of a four. You could have squares that call up different grids, and so on.
The point is that this could all be done with a thumb, and basic letters and numbers could probably be done without looking with very little practice.