The talkgroup issue of having multiple trunked systems in one scanner bank relates to TGIDs and their nicknames. I can put 10 different MOT systems in one bank of my old Pro-92 (assuming 5 channels per system), but if 01648 is police dispatch for one system and animal control for another and telemetry for a third, I would have to listen to all or none. This is separate from and in addition to potential issues with control channels.
But as DJ88 noted, the original poster has a BC246T, which has dynamic memory and can scan something upwards of 100 trunked systems at the same time, each in its own "bank." So for Ian, it probably doesn't make sense to put more than one system in one "bank" (with dynamic memory, the terminology varies a bit).
We're getting a little off-topic here. For additions, I'd suggest aviation and maybe trains. If you explore a little, you may find some interesting business channels, either conventional or trunked (most often, LTR). And lastly, Philly or NYC if you travel there often or even occassionally.