If you can afford it then different kit for different conditions is the way to go. If you buy general use kit you'll find that there is always something lacking, that said i can't afford it myself so much of my kit is general use. There are some things you can get or consider to make your kit more versatile though.
Gore Tex boot liners. I got a pair through my brother who is in the army, they are like socks, and mean my lightweight, barely water resistant boots, which are perfect on hot summer days, can now be used in spring showers and autumn too.
Extensive compression straps on your bag mean you can just buy one big enough for long winter trips, and keep it stable for those smaller summer loads. (Check this carefully, just having compression straps doesn't mean they are actually useful!!)
Gaiters are often more useful than waterproof trousers, unless its quite heavy rain, and/or high winds. (your trousers get wetter from the water already on the ground) and you won't get too hot. That said windproof over trousers can make a big difference keeping you warm in winter.
Sooner than buy a thick coat, by a thinner coat that will be comfortable (not too warm) in summer/spring, but a little larger than you would normally (be sensible though) just enough room for an extra fleece underneath when its cold. While on the subjects of coats, getting one with a good hood can cost a fortune, far better and more comfortable IMO to buy a separate waterproof hat (with a good rim for shade in summer) and not worry about the hood on your coat.
A water sytem, that fits down the back of your bag, is more convenient, and a more efficient use of space than bottles, feels more stable too. (IMO)
I personally find sleeping bag liners very effective at adding warmth, i can use my 1 season bag well into autumn with my micro fleece liner (that's me, you might not get as long out of it, but I'm sure it will extend the use of your sleeping bag to a notable extent) Plus its easier to clean, lighter, and takes less room than having a warmer bag.