Love the guy but Dragon Age as a setting would be a much more natural fit for him and his style, imo; particularly as a very well known eccentric old school British voice-actor the roles he could play are virtually endless in medieval fantasy - priest, judge, merchant, scholar, wizard etc. while elderly British actors are a bit typecast in science fiction.
I think the main problem with having a well known voice talent in a science fiction property like Mass Effect is that while alien roles tend to be more interesting ones if he isn't limited to a human role you could start to loose the essence of his signature sound/style when you start messing around with filters, etc. to make him sound more alien, i.e. recognisably Krogan, Salarian or Turian, etc. - rendering moot the entire point of hiring him in the first place. Alien females however tend to sound like humans in Mass Effect for some reason.
I would personally be interested to see if he could pull off a main antagonist role in a video game without it coming off as campy or cheesy - if he hasn't already. If I had to pick any particular alien race for him (or somebody like him) to depict I'd like to see if he could pull off a legendary Krogan Okeer type figure (as bizarre as that might be/sound); but Batarian might mesh the best with that style/sound while keeping some of his recognizable character and distinctive sound of his which might otherwise be lost in the process if you have to change the pitch/tempo too much. If not as a main antagonist, then maybe as some kind of peripheral ex-military commander, mercenary/warlord, doctor, philanthropist or probably more likely as some kind of riff on the mad scientist trope. How about a Krogan Billionaire like say, Mr. Thax?
I may be wrong about this but in order to keep the alien voices consistent aren't the voice actors who play certain races predominantly from the same region/area, i.e. Krogan voice actors from Texas?
In a human role - I can't really see him as a core squad/companion character unfortunately given his age as the main human squad members tend to be love interests, possibly as a crew member but that might be a waste of his talent and the v.o. budget. Probably more likely to have a minor cameo as a Councillor or in an Alliance leadership type role if he was offered a human part. Personally I think having a human antagonist even if it was Brian Blessed would be a bit of a waste of the source material.