I am trying to design a speller paradigm which can have better acceptability and accuracy. At a same time I need to know what are the critical factors that should be met during the design of a paradigm.
A very important factor is the ease of operation from the point of view of the user. For example, the P300 speller is very reliable but the flashing stimuli are annoying for most users and it is not easy to use this speller for an extended period. Some users actually prefer a slow rate of seplling (e.g., 20 sec per letter) to allow for prolonged usage. The important questions here are what is the typical duration of a session going to be and what is the required spelling rate? These are competing factors and the design may involve some trade-off between them.
Other factors are:
The number of electrodes to be used - is there a limitation?
Training - is there a constraint on the duration of the training period before actual use?
Reliability of the relevant EEG features - are the EEG features to be used easy to extract in different subjects? are they consistent for the same subject across different sessions?
Individual differences among users - what modifications can be made to best adapt the paradigm for each individual user?
Obviously, there are many more factors. There are quite a few good review papers on BCI as well as books that might be useful for you. They discuss these issues in much detail.
Assuming you are using EEG, one crucial aspect of the paradigm is to limit/reduce the noise, both at the physical level (e.g. the electrode-scalp contact) and at the digital-processing level.