I know passengers won't care as to them a bus is a bus and is essentially just a box on four wheels but as an enthusiast I'd like to see some variety. The Darts are all from the same batch whereas 42109/115/723 are fairly unique.
Streetlites may well be the preferred choice but if a city is covered with them it'll look so boring. Or at least do what Reading do and have them all colour coordinated.
Slightly contradicting what I said earlier, Bristol does have B7RLEs, Geminis, ALX400s, Darts, Streetlites, B10Ls (or B10Bs, whatever are used on the 78/79), Enviro 400s and Enviro 200s, although in general it is mainly the Geminis and Streetlites that dominate the streets of Bristol.
I also like the ex-Southampton batch of Tridents that have recently moved to Hengrove.
However, when I say a variety, I do NOT mean to the extent of the 319. One day I saw FIVE different types of buses on there, which was just ridiculous. I do wish there was a fixed fleet on there. Some of the tiny Darts (4255x range) were on there the other day and they are just FAR too small for such a route. The 319 to me is a mixture of different routes all fused together, so a moderately sized bus would be nice for the route. It's like putting a Pacer or a Sprinter on the Aberdeen to Penzance XC service!
I personally think the 319 should be a decker route, as it's constantly busy. Either full 100% operation from the current Tridents there (32282, 32284, etc), Geminis or a batch of 6 or 7 Enviro 400s (like the X1 has). I would even like to see branded vehicles on it, the same view also going for other routes in Bristol such as the 1/2, 8/9, 42/43/44/45, 70/71/72 and the 75/76.